U.S. patent application number 11/732490 was filed with the patent office on 2007-10-18 for production of three-dimensional objects by use of electromagnetic radiation.
This patent application is currently assigned to Z Corporation. Invention is credited to Amir Alam, Benjamin Berrington, James F. Bredt, Tom Davidson, Eugene Giller, Derek X. Williams.
Application Number | 20070241482 11/732490 |
Document ID | / |
Family ID | 38458364 |
Filed Date | 2007-10-18 |
United States Patent
Application |
20070241482 |
Kind Code |
A1 |
Giller; Eugene ; et
al. |
October 18, 2007 |
Production of three-dimensional objects by use of electromagnetic
radiation
Abstract
Process, materials, and equipment for producing
three-dimensional objects from a particulate material by melting
and adhering, for example, by fusion or sintering, portions of the
particulate material.
Inventors: |
Giller; Eugene; (Needham,
MA) ; Bredt; James F.; (Watertown, MA) ;
Davidson; Tom; (Arlington, MA) ; Williams; Derek
X.; (Berwick, ME) ; Alam; Amir; (Marlborough,
MA) ; Berrington; Benjamin; (Brighton, MA) |
Correspondence
Address: |
GOODWIN PROCTER LLP;PATENT ADMINISTRATOR
EXCHANGE PLACE
BOSTON
MA
02109-2881
US
|
Assignee: |
Z Corporation
Burlington
MA
|
Family ID: |
38458364 |
Appl. No.: |
11/732490 |
Filed: |
April 3, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60789758 |
Apr 6, 2006 |
|
|
|
Current U.S.
Class: |
264/494 ; 522/71;
525/438; 525/440.05 |
Current CPC
Class: |
B29K 2105/16 20130101;
B29C 64/165 20170801; B29K 2101/10 20130101; G03G 15/224 20130101;
B29K 2101/12 20130101; C04B 2111/00181 20130101 |
Class at
Publication: |
264/494 ;
522/071; 525/438; 525/440.05 |
International
Class: |
B29C 71/04 20060101
B29C071/04; B29C 35/08 20060101 B29C035/08 |
Claims
1. A material system for three dimensional printing comprising: a
granular material including: a first particulate adhesive selected
from the group consisting of a thermoset material and a
thermoplastic material; and an absorber capable of being heated
upon exposure to electromagnetic energy sufficiently to bond the
granular material, wherein a static and a dynamic friction
coefficient of the granular material possess a relationship defined
by a Bredt parameter having a value in excess of 0.1.
2. The material system of claim 1, wherein the thermoplastic
material is selected from the group consisting of
polyphenylsulfone, polyacrylonitrile, polycondensates of
urea-formaldehyde, polyolefins, cyclic polyolefins, polyvinyl
butyral, polyvinyl chlorides, acrylics, ethyl cellulose,
hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose,
cellulose acetate, hydroxypropylmethyl cellulose,
hydroxybutylmethyl cellulose, hydroxyethylmethyl cellulose,
ethylhydroxyethyl cellulose, cellulose xanthate, and combinations
and copolymers thereof.
3. The material system of claim 1, wherein the thermoset material
is selected from the group consisting of epoxy with aromatic
amines, epoxy with aliphatic amines, amides, acid anhydrides,
multifunctional acids; isocyanate/amine, isocyanate/alcohol,
unsaturated polyesters, vinyl esters, unsaturated polyester and
vinyl ester blends, unsaturated polyester/urethane hybrid resins,
polyurethane/urea, reactive dicyclopentadiene resin, reactive
polyamides, polyester sulfones, a moisture-curable hot melt
polyurethane, pulverized/encapsulated epoxy in combination with
pulverized dicyanamide, at least one of high molecular-weight
polyols, high molecular-weight polyamines, and high
molecular-weight polythiols in combination with at least one of
isocyanates, diacids, polyacids, and multifunctional acid
anhydrides, and combinations and copolymers thereof.
4. The material system of claim 1, wherein the granular material
further comprises a second adhesive material.
5. The material system of claim 1, wherein the first adhesive is at
least partially soluble in a fluid applied to the granular material
during three dimensional printing.
6. The material system of claim 5, wherein the at least partially
soluble adhesive is selected from the group consisting of polyvinyl
alcohol, sulfonated polyester polymer, sulfonated polystyrene,
octylacrylamide/acrylate/butylaminoethyl methacrylate copolymer,
acrylates/octylacrylamide copolymer, polyacrylic acid, polyvinyl
pyrrolidone, styrenated polyacrylic acid, polyethylene oxide,
sodium polyacrylate, sodium polyacrylate copolymer with maleic
acid, polyvinyl pyrrolidone copolymer with vinyl acetate, butylated
polyvinylpyrrolidone, polyvinyl alcohol-co-vinyl acetate, and
combinations and copolymers thereof.
7. The materials system of claim 1, wherein the granular material
further comprises a filler.
8. The materials system of claim 7, wherein the filler is
inert.
9. The materials system of claim 8, wherein the inert filler is an
inorganic filler selected from the group consisting of plaster,
terra alba, bentonite, calcium silicate, calcium phosphate,
magnesium silicate, magnesium phosphate, aluminum oxide, aluminum
hydroxide, limestone, dolomite, wollasonite, mica, glass fiber,
glass powder, cellulose fiber, silicon carbide fiber, graphite
fiber, aluminosilicate fiber, mineral fiber, and combinations
thereof.
10. The materials system of claim 8, wherein the inert filler is an
organic filler selected from the group consisting of starch,
modified starch, maltodextrin, cellulose, polypropylene fiber,
polyamide flock, rayon, polyvinyl alcohol fiber, sugars and sugar
alcohols, carbohydrates, and combinations thereof.
11. The materials system of claim 7, wherein the filler comprises a
highly reflective particulate material.
12. The materials system of claim 11, wherein the filler is
selected from the group consisting of a metal oxide particle, high
refractive index glass, sapphire; metal dust, and a particle
comprising at least two materials with significantly different
refractive indices.
13. The materials system of claim 12, wherein the metal oxide
particle is selected from the group consisting of titania and
zirconia.
14. The materials system of claim 12, wherein the metal dust is
selected from the group consisting of aluminum and steel.
15. The materials system of claim 12, wherein the particle
comprising at least two materials is selected from group consisting
of a hollow glass bead and a core/shell glass bead.
16. The materials system of claim 12, wherein the metallic oxide is
selected from the group consisting of titanium dioxide, aluminum
oxide, magnesium oxide, zinc oxide, amorphous silica, fumed silica,
and crystalline silica.
17. The materials system of claim 7, wherein the filler is
chemically reactive with a fluid applied to the granular material
during three dimensional printing to form a partly bonded structure
to reduce contraction or expansion of the first particulate
adhesive.
18. The materials system of claim 7, wherein the filler is
chemically reactive with a fluid applied to the granular material
during three dimensional printing to generate heat that causes the
first particulate adhesive to bond form a solid article.
19. The materials system of claim 7, wherein the filler comprises
an active filler selected from the group consisting of plaster,
bentonite, sodium silicate, salt, Portland cement, magnesium
phosphate cement, magnesium chloride cement, magnesium sulfate
cement, zinc phosphate cement, calcium phosphate cement, zinc
oxide-eugenol cement, and combinations thereof.
20. The materials system of claim 7, wherein the granular material
further comprises a plasticizer selected to lower a melting point
of the first adhesive material.
21. The materials system of claim 20, wherein the plasticizer is
selected from the group consisting of mineral oils; phthalates,
phosphates, adipates-dioctyl phthalate, dioctyl adipate, diisononyl
phthalate, dibenzyl phthalate, dipropylene glycol dibenzoate,
triaryl phosphate ester; epoxidized soybean oil, glycerol,
propylene glycol, urea, ethoxylated glycerol, butanediol,
pentanediol, hexanediol, erythritol, xylitol, sorbitol, and
combinations thereof.
22. The materials system of claim 7, wherein the granular material
further comprises a plasticizer selected to lower a flow viscosity
of the first adhesive material upon melting.
23. The materials system of claim 22, wherein the plasticizer is
selected from the group consisting of mineral oils; phthalates,
phosphates, adipates-dioctyl phthalate, dioctyl adipate, diisononyl
phthalate, dibenzyl phthalate, dipropylene glycol dibenzoate,
triaryl phosphate ester; epoxidized soybean oil, glycerol,
propylene glycol, urea, ethoxylated glycerol, butanediol,
pentanediol, hexanediol, erythritol, xylitol, sorbitol, and
combinations thereof.
24. A process for producing a three-dimensional object, the process
comprising the steps of: a) providing a first layer of a dry
particulate material; b) selectively applying at least a first
absorber to a region of the first layer of the dry particulate
material, wherein the region is selected in accordance with a cross
section of the three-dimensional object; c) treating the first
layer with electromagnetic energy selected from the group
consisting of spatially incoherent, polychromatic, and
phase-incoherent, the electromagnetic energy being absorbed by the
absorber to heat the treated region so as to at least one of melt
and sinter the dry particulate material disposed in the region; and
d) cooling the first layer.
25. The process of claim 24, wherein the electromagnetic energy is
applied by a source selected from the group consisting of an
unfocused laser of wavelength from 100 nm to 1 mm; a radiant heater
or emission lamp radiation comprising at least one of visible (400
nm-750 nm), IR-A (750 nm-1400 nm) and IR-B (1400-5000 nm)
radiation; and an oscillating magnetic field producing
electromagnetic induction.
26. The process of claim 24, wherein the absorber is applied as a
component in a first fluid, the process further comprising causing
a chemical reaction to occur between reactive components in the
powder, wherein the fluid stimulates the reaction.
27. The process of claim 26, further comprising: controlling a
temperature of the region of the first layer of the particulate
material by depositing a second fluid having a boiling point below
a bonding point of the particulate material, wherein the first
fluid is deposited in a first pattern and the second fluid is
deposited in a second pattern surrounding the first pattern defined
by the first fluid.
28. The process of claim 24, further comprising at least one of
melting and sintering the first region of the dry particulate
material to a second region disposed in a second layer of dry
particulate material situated proximate the first layer.
29. The process of claim 28, wherein the second region comprises a
second absorber.
30. The process of claim 29, wherein the first absorber and the
second absorber are the same.
31. The process of claim 29, wherein the first absorber and the
second absorber are different.
32. The process of claim 24, further comprising: selectively
applying a second fluid to the region of first layer of the
particulate material, the second fluid comprising a reactive
monomer and a photoinitiator, said reactive monomer being
solidified by the application of electromagnetic radiation.
33. The process of claim 24, further comprising: removing
unsintered particulate material; depositing a layer of a second
particulate material in a second region, wherein the second region
excludes the first region; sintering or otherwise bonding said
second particulate material by at least one of application of heat
and a solvent action of a printed fluid to form a support structure
that is contiguous with the region of the first layer of the dry
particulate material powder and with a movable platform defining a
build surface for the three-dimensional object.
34. A process for producing a three-dimensional object, the process
comprising the steps of: a) providing a first layer of a dry
particulate material; b) selectively applying a first fluid to a
region of the first layer of the dry particulate material, wherein
the region is selected in accordance with a cross section of the
three-dimensional object; c) causing a chemical reaction to occur
with a first reactive component of the dry particulate material,
and releasing energy by this reaction in the form of heat to at
least one of melt and sinter the region of the particulate material
containing the fluid; and d) cooling the layer.
35. The process of claim 34, wherein the chemical reaction occurs
between the first reactive component and the fluid.
36. The process of claim 35, wherein the dry particulate material
comprises a second reactive component, and the chemical reaction
occurs between the first and second reactive components, and is
stimulated by the fluid.
37. The process of claim 34, further comprising: at least one of
melting and sintering the region comprising the fluid to a second
region of a second layer of dry particulate material disposed
proximate the first layer.
38. The process of claim 34, further comprising: controlling a
temperature of the region of the first layer of the particulate
material by depositing a second fluid having a boiling point below
a bonding point of the particulate material, wherein the first
fluid is deposited in a first pattern and the second fluid is
deposited in a second pattern surrounding the first pattern defined
by the first fluid.
39. The process of claim 34, further comprising: selectively
applying a second fluid to the region of first layer of the
particulate material, the second fluid comprising a reactive
monomer and a photoinitiator, said reactive monomer being
solidified by the application of electromagnetic radiation.
40. The process of claim 34, further comprising: removing
unsintered particulate material; depositing a layer of a second
particulate material in a second region, wherein the second region
excludes the first region; sintering or otherwise bonding said
second particulate material by at least one of application of heat
and a solvent action of a printed fluid to form a support structure
that is contiguous with the region of the first layer of the dry
particulate material powder and with a movable platform defining a
build surface for the three-dimensional object.
41. The process of claim 40, further comprising: controlling a
temperature of support structure by cooling the moveable platform
and allowing heat to conduct from the three-dimensional object
formed by the first material and through the support structure
formed by the second material.
42. A machine for three-dimensional printing comprising: a printing
device; a spreading mechanism; a heat source; and a temperature
controller, the temperature controller including at least one of a
non-contact thermometer, a software algorithm that responds to said
thermometer, a heat-transfer surface disposed within a build box,
and a cooling mechanism that operates by flowing air over a powder
surface.
43. A kit for three dimensional printing, the kit comprising: a
fluid comprising a first solvent, a second solvent, and an
absorber; and a first particulate adhesive material selected from
the group consisting of a thermoplastic material and a thermoset
material, wherein the first solvent has a boiling point above at
least one of a sintering point and a melting point of the first
particulate adhesive material.
44. The kit of claim 43, wherein the first solvent is selected from
the group consisting of ethanol, isopropanol, n-propanol, methanol,
n-butanol, a glycol, an ester, a glycol-ether, a ketone, an
aromatic, an aliphatic, an aprotic polar solvent, a terpene, an
acrylate, a methacrylate, a vinylether, an oxetane, an epoxy, a low
molecular weight polymer, carbonate, n-methylpyrrolidone, acetone,
methyl ethyl ketone, dibasic esters, ethyl acetate, dimethyl
sulfoxide, dimethyl succinate, and combinations thereof.
45. The kit of claim 43, wherein the second solvent has a second
boiling point lower than a melting point of the first particulate
adhesive material.
46. The kit of claim 43, wherein the second solvent has a second
boiling point lower than a sintering point of the first particulate
adhesive material.
47. The kit of claim 43, wherein the second solvent comprises
water.
48. The kit of claim 43, wherein the absorber is adapted to absorb
electromagnetic radiation at a wavelength selected from a range of
100 nm to 1 mm.
49. The kit of claim 43 wherein the absorber is adapted to suscept
an oscillating magnetic field and heat by electromagnetic induction
and is selected from the group consisting of a metal, granular
carbon, a polar organic compound, an aqueous solution of an ionic
substance, and a mineral having a high conductivity.
50. The kit of claim 43, wherein the fluid further comprises a
flowrate enhancer.
51. The kit of claim 43, wherein the fluid further comprises a
reactive monomer.
Description
RELATED APPLICATION
[0001] This application claims the benefit of and priority to U.S.
Provisional Application Ser. No. 60/789,758 filed Apr. 6, 2006, the
entire disclosure of which is hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] The invention relates to a process for producing
three-dimensional objects from a particulate material by melting
and adhering, for example, by fusion or sintering, portions of the
particulate material; the heat needed for the bonding of the
particulate material may be generated by a laser, or a non-oriented
and/or non-monochromatic and/or non-coherent energy source of
wavelength from 100 nm to 1 mm or by electromagnetic induction by
way of an absorber, and transferred by way of the absorber to the
subregions of the particulate material. Additionally, heat may be
supplied by a chemical reaction between reactive components in the
fluid, particulate material, or both.
BACKGROUND
[0003] There is a need for the rapid production of prototypes.
Stereolithography, a process that fabricates models in a bath of
liquid photopolymer, needs complicated support structures to retain
the solidified material in the liquid bath, and the resultant
prototypes have relatively poor mechanical properties, attributable
to a limited number of starting materials.
[0004] Another process for rapid prototyping is selective laser
sintering (SLS), which has become widespread. In this process,
granulated polymers in a chamber are selectively irradiated briefly
with a laser beam, resulting in the melting of the particles on
which the laser beam falls. The molten particles coalesce and
solidify again relatively rapidly to give a solid mass. Complex
three-dimensional bodies can be produced simply and rapidly by this
process by repeatedly applying fresh layers of polymer and
irradiating these layers.
[0005] The process of laser-sintering (rapid prototyping) to form
moldings composed of pulverulent polymers has been described in the
literature. The SLS processes typically have the disadvantage of
requiring a focused laser beam. The laser, functioning as energy
source, may be expensive and sensitive, as is the optical equipment
needed for the production and focusing of the laser beam, such as
lenses, expanders, and deflector mirrors.
[0006] Other processes have been developed for rapid prototyping,
but have not been introduced to the market. For example, a
previously developed process for producing prototypes is based on
printed inhibitors that prevent sintering in selected regions, the
unprinted regions later being sintered by heat. This process can
operate without any complicated laser technology. A disadvantage of
this process is that the surrounding particulate material that was
not sintered still contains the inhibitor, and therefore, cannot be
recycled. In addition, this process may require the development of
new software, specifically because it is the surrounding area that
is printed, and not, as in other cases, the cross section of the
part. For undercuts and changes in cross section, inhibitors are
typically deposited over a large surface area. In addition, there
is a risk of heat build up in the developing prototype.
[0007] The use of microwave radiation for the melting of polymers
is described in U.S. Pat. No. 5,338,611. In this process,
pulverulent polymers and nano-scale carbon black are used. However,
this reference does not describe the production of prototypes.
Reference DE 197 27 677 describes a method of generating prototypes
by exposing selected regions of layers of particulate material to a
focused microwave beam. The controlled microwave beam bonds the
exposed particulate material within a layer, and also bonds this
particulate material to the particulate material in the layer
situated thereunder. Bonding takes place via adhesive bonding,
sintering, or fusion. This process may also require complicated
technology to ensure that the microwave radiation reaches only the
selected regions.
[0008] Prototyping methods that use focused sources of radiation
are relatively expensive and complicated and may require frequent
maintenance. The microwave process ameliorates some of these
problems by using a cheaper source of radiation that does not need
to be focused. Microwave radiation, however, may be troublesome to
contain, particularly in situations where electronic components are
in close proximity to the microwave source.
[0009] Another method for creating three dimensional prototypes
employs infrared light to bond thermoplastic and thermoset
particulate materials in a sequence of patterned layers. In this
process, the pattern for each layer is formed into a mask on a
glass plate by an electrostatic imaging process. The glass plate
used to filter infrared light into a programmed pattern over a
layer of loose particulate material. While this method possesses
the advantage of being relatively fast, it may require the creation
of a collimated beam of light that can form a sharp image on the
surface of a build area, i.e., the region in the machine where the
build material is solidified by an imaging process to form a solid
article. The light rays in this beam are typically oriented
parallel to one another in order to cast sharply defined patterns
of radiation on the surface of the particulate material. A special
collimating filter may be included with the light source in order
to achieve this, and the dissipation of the large fraction of
filtered-out radiation is left unresolved. Further, this approach
poses little opportunity to control the undesirable flow of heat by
direct conduction from bonded regions into adjacent unbound regions
within the particulate material bed.
[0010] See also DE 10 2004 020 452.7, WO 2005/105412, and U.S.
Patent Publication No. 2004/0232583 A1, incorporated herein by
reference.
SUMMARY OF THE INVENTION
[0011] In an aspect, the invention features a process for the
production of three-dimensional objects using a simple, low-cost
apparatus that is substantially unsusceptible to failure.
[0012] The components of the apparatus are preferably of robust
design. In another aspect, the invention features an object
produced by this process. In yet another aspect, the invention
features the aforementioned apparatus for the production of
three-dimensional objects.
[0013] More particularly, a process for producing a
three-dimensional object may include the following steps:
a) providing a layer of a dry particulate material,
b) selectively applying at least a first absorber to one or more
regions of the particulate material, wherein the one or more
regions are selected in accordance with a cross section of the
three-dimensional object,
c) optionally causing a chemical reaction to occur between reactive
components in the particulate material, such reaction stimulated by
a liquid component otherwise serving as a vehicle for deposition of
the absorber,
[0014] d) treating the layer with electromagnetic energy that may
be spatially incoherent (i.e., unfocused and uncollimated),
polychromatic, or phase-incoherent, using an unfocused laser of
wavelength from 100 nm to 1 mm; radiant heaters or emission lamps
applying radiation comprising visible (400 nm-750 nm), IR-A (750
nm-1400 nm) or IR-B (1400-5000 nm) radiation; or by electromagnetic
induction through an oscillating magnetic field to melt, sinter, or
bond through a thermally activated chemical reaction the one or
more regions containing the first absorber to the layer of
particulate material, and, optionally, to melt, sinter, or bond
through a thermally activated chemical reaction the one or more
regions containing the first absorber with other regions located in
one or more substrate layers situated thereunder, thereabove, or
combinations thereof,
wherein the other regions optionally contain a second absorber, and
wherein the first absorber and the second absorber are the same or
different, and
[0015] e) cooling the layer, passively or actively.
[0016] Additional aspects of the invention include the production
of three-dimensional objects prepared according to the described
process, and an apparatus for producing three-dimensional
objects.
[0017] It has been found that three-dimensional objects may be
produced from particulate materials bonded relatively simply by
means of low-cost lasers or non-laser sources of electromagnetic
energy, the radiation from which is not spatially coherent (in
other words, neither focused nor collimated) and/or from a diffuse
source and may emit a range of wavelengths outside of the microwave
range, by applying one or more absorbers to those regions to be
bonded in a layer of a particulate material. The particulate
material absorbs radiation only poorly or not at all, while the
absorber(s) absorbs the radiation and passes the energy absorbed in
the form of heat to the particulate material surrounding the
absorber(s). This results in the melting and fusing of the
particulate material and where appropriate, the melting and fusing
of the particulate material to another layer situated thereunder or
thereover. The susceptible regions may be fused or sintered. The
absorber may be applied using a printing head, similar to that of
an inkjet printer.
[0018] The absorber process described here provides a reasonably
accurate way to deliver heat to a printed layer in a 3D Printer.
The class of printers addressed by aspects of the invention are
generally those in which a dry particulate build material is
treated with a liquid deposited in a cross-section of an article to
be built, this liquid engendering a solidification or bonding
mechanism in the particulate build material. Suitable printers are
described, for example, in U.S. Pat. No. 5,204,055, incorporated
herein by reference in its entirety.
[0019] Melting and fusion or sintering of thermoplastic particulate
materials is only one method by which heat can form a solid
structure in a printed layer. For example, certain reactive
mixtures of monomers or oligomers are stable as particulate
materials at room temperature but melt and form a crosslinked
polymer when exposed to heat. These "thermoset" materials are more
compatible with inert fillers than are thermoplastic materials
because they have a lower viscosity on melting, and therefore they
can flow a greater distance and merge filler particles more
completely before solidifying.
[0020] Additionally, some reactive combinations of particulate
materials, such as cements, anhydrous salts, and organic
anhydrides, may react with the water in the fluid only under the
action of heat. For these types of chemical reactions, the
radiation may be thought of as providing the activation energy to
initiate a chemical reaction between the fluid and certain reactive
components in the particulate material.
[0021] Thermoplastic and thermoset particulate materials are
available as commercial formulations for other processes, such as
powder-coating for metal finishing. Many, if not all, currently
available formulations may be unsuitable for 3D Printing because
the blends sold for other purposes may not fall within a relatively
narrow range of particle size and frictional characteristics
preferred to enable 3D Printing. A commercial blend of
powder-coating material may be rendered useable in the described
processes only by further processing, such as by milling and
classifying the particulate material, or by adding one or several
particulate or liquid additives, or by aggregating or coating
thermoplastic compositions onto grains of inert fillers or
combinations thereof. The additives may additionally provide some
improvement in performance, such as stiffness, but the frictional
characteristics of the blend are vitally important in determining
their handling properties during spreading and therefore determine
the usability of a particular formula.
[0022] In an aspect, the invention features a material system for
three dimensional printing. The material system includes a granular
material that includes a first particulate adhesive including a
thermoset material and/or a thermoplastic material. The material
system also includes an absorber capable of being heated upon
exposure to electromagnetic energy sufficiently to bond the
granular material. A static and a dynamic friction coefficient of
the granular material possess a relationship defined by a Bredt
parameter having a value in excess of 0.1.
[0023] One or more of the following features may be included. The
thermoplastic material may include or consist of polyphenylsulfone,
polyacrylonitrile, polycondensates of urea-formaldehyde,
polyolefins, cyclic polyolefins, polyvinyl butyral, polyvinyl
chlorides, acrylics, ethyl cellulose, hydroxyethyl cellulose,
hydroxypropyl cellulose, methyl cellulose, cellulose acetate,
hydroxypropylmethyl cellulose, hydroxybutylmethyl cellulose,
hydroxyethylmethyl cellulose, ethylhydroxyethyl cellulose,
cellulose xanthate, and combinations and copolymers thereof.
[0024] The thermoset material is may include or consist of epoxy
with aromatic amines, epoxy with aliphatic amines, amides, acid
anhydrides, multifunctional acids; isocyanate/amine,
isocyanate/alcohol, unsaturated polyesters, vinyl esters,
unsaturated polyester and vinyl ester blends, unsaturated
polyester/urethane hybrid resins, polyurethane/urea, reactive
dicyclopentadiene resin, reactive polyamides, polyester sulfones, a
moisture-curable hot melt polyurethane, pulverized/encapsulated
epoxy in combination with pulverized dicyanamide, at least one of
high molecular-weight polyols, high molecular-weight polyamines,
and high molecular-weight polythiols in combination with at least
one of isocyanates, diacids, polyacids, and multifunctional acid
anhydrides, and combinations and copolymers thereof.
[0025] The granular material may include a second adhesive
material.
[0026] The first adhesive may be at least partially soluble in a
fluid applied to the granular material during three dimensional
printing. The at least partially soluble adhesive may include or
consist of polyvinyl alcohol, sulfonated polyester polymer,
sulfonated polystyrene, octylacrylamide/acrylate/butylaminoethyl
methacrylate copolymer, acrylates/octylacrylamide copolymer,
polyacrylic acid, polyvinyl pyrrolidone, styrenated polyacrylic
acid, polyethylene oxide, sodium polyacrylate, sodium polyacrylate
copolymer with maleic acid, polyvinyl pyrrolidone copolymer with
vinyl acetate, butylated polyvinylpyrrolidone, polyvinyl
alcohol-co-vinyl acetate, and combinations and copolymers
thereof.
[0027] The granular material may include a filler. The filler may
be inert. The inert filler may include or consist of plaster, terra
alba, bentonite, calcium silicate, calcium phosphate, magnesium
silicate, magnesium phosphate, aluminum oxide, aluminum hydroxide,
limestone, dolomite, wollasonite, mica, glass fiber, glass powder,
cellulose fiber, silicon carbide fiber, graphite fiber,
aluminosilicate fiber, mineral fiber, and combinations thereof.
[0028] The inert filler may include or consist of an organic filler
such as starch, modified starch, maltodextrin, cellulose,
polypropylene fiber, polyamide flock, rayon, polyvinyl alcohol
fiber, sugars and sugar alcohols, carbohydrates, and combinations
thereof.
[0029] The filler may include or consist of a highly reflective
particulate material, such as a metal oxide particle, high
refractive index glass, sapphire; metal dust, and/or a particle
comprising at least two materials with significantly different
refractive indices. The metal oxide particle may include titania
and/or zirconia. The metal dust may include or consist of aluminum
and steel. The particle comprising at least two materials may be a
hollow glass bead and/or a core/shell glass bead. The metallic
oxide may include or consist of titanium dioxide, aluminum oxide,
magnesium oxide, zinc oxide, amorphous silica, fumed silica, and
crystalline silica.
[0030] The filler may be chemically reactive with a fluid applied
to the granular material during three dimensional printing to form
a partly bonded structure to reduce contraction or expansion of the
first particulate adhesive.
[0031] The filler may be chemically reactive with a fluid applied
to the granular material during three dimensional printing to
generate heat that causes the first particulate adhesive to bond
form a solid article.
[0032] The filler may include an active filler, such as plaster,
bentonite, sodium silicate, salt, Portland cement, magnesium
phosphate cement, magnesium chloride cement, magnesium sulfate
cement, zinc phosphate cement, calcium phosphate cement, zinc
oxide-eugenol cement, and combinations thereof.
[0033] The granular material may include a plasticizer selected to
lower a melting point of the first adhesive material. The
plasticizer may include or consist of mineral oils; phthalates,
phosphates, adipates-dioctyl phthalate, dioctyl adipate, diisononyl
phthalate, dibenzyl phthalate, dipropylene glycol dibenzoate,
triaryl phosphate ester; epoxidized soybean oil, glycerol,
propylene glycol, urea, ethoxylated glycerol, butanediol,
pentanediol, hexanediol, erythritol, xylitol, sorbitol, and
combinations thereof.
[0034] The granular material may include a plasticizer selected to
lower a flow viscosity of the first adhesive material upon melting.
The plasticizer may include or consist of mineral oils; phthalates,
phosphates, adipates-dioctyl phthalate, dioctyl adipate, diisononyl
phthalate, dibenzyl phthalate, dipropylene glycol dibenzoate,
triaryl phosphate ester; epoxidized soybean oil, glycerol,
propylene glycol, urea, ethoxylated glycerol, butanediol,
pentanediol, hexanediol, erythritol, xylitol, and sorbitol.
[0035] In another aspect, the invention features a process for
producing a three-dimensional object, the process including the
following steps: a) providing a first layer of a dry particulate
material; b) selectively applying at least a first absorber to a
region of the first layer of the dry particulate material, the
region being selected in accordance with a cross section of the
three-dimensional object; c) treating the first layer with
electromagnetic energy including at least one of spatially
incoherent, polychromatic, and phase-incoherent, the
electromagnetic energy being absorbed by the absorber to heat the
treated region so as to melt or sinter the dry particulate material
disposed in the region; and d) cooling the first layer.
[0036] One or more of the following features may be included. The
electromagnetic energy may be applied by a source selected from the
group consisting of an unfocused laser of wavelength from 100 nm to
1 mm; a radiant heater or emission lamp radiation comprising at
least one of visible (400 nm-750 nm), IR-A (750 nm-1400 nm) and
IR-B (1400-5000 nm) radiation; and an oscillating magnetic field
producing electromagnetic induction. The absorber may be applied as
a component in a first fluid, the process further comprising
causing a chemical reaction to occur between reactive components in
the powder, wherein the fluid stimulates the reaction.
[0037] The process may include melting or sintering the first
region of the dry particulate material to a second region disposed
in a second layer of dry particulate material situated proximate
the first layer. The second region may include a second absorber.
The first absorber and the second absorber are the same or
different.
[0038] In yet another aspect, the invention features a process for
producing a three-dimensional object, the process including the
steps of: a) providing a first layer of a dry particulate material;
b) selectively applying a first fluid to a region of the first
layer of the dry particulate material, wherein the region is
selected in accordance with a cross section of the
three-dimensional object; c) causing a chemical reaction to occur
with a first reactive component of the dry particulate material,
and releasing energy by this reaction in the form of heat to melt
or sinter the region of the particulate material containing the
fluid; and d) cooling the layer.
[0039] The chemical reaction may occur between the first reactive
component and the fluid. The dry particulate material may include a
second reactive component, and the chemical reaction may occur
between the first and second reactive components, and may be
stimulated by the fluid.
[0040] The process may include melting or sintering the region
comprising the fluid to a second region of a second layer of dry
particulate material disposed proximate the first layer. The
process may include controlling a temperature of the region of the
first layer of the particulate material by depositing a second
fluid having a boiling point below a bonding point of the
particulate material, and the first fluid may be deposited in a
first pattern and the second fluid may be deposited in a second
pattern surrounding the first pattern defined by the first
fluid.
[0041] The process may include selectively applying a second fluid
to the region of first layer of the particulate material, the
second fluid comprising a reactive monomer and a photoinitiator,
the reactive monomer being solidified by the application of
electromagnetic radiation.
[0042] The process may include removing unsintered particulate
material; depositing a layer of a second particulate material in a
second region, wherein the second region excludes the first region;
sintering or otherwise bonding the second particulate material by
at least one of application of heat and a solvent action of a
printed fluid to form a support structure that is contiguous with
the region of the first layer of the dry particulate material
powder and with a movable platform defining a build surface for the
three-dimensional object.
[0043] A temperature of support structure may be controlled by
cooling the moveable platform and allowing heat to conduct from the
three-dimensional object formed by the first material and through
the support structure formed by the second material.
[0044] In another aspect, the invention features a machine for
three-dimensional printing including a printing device; a spreading
mechanism; a heat source; and a temperature controller, the
temperature controller including at least one of a non-contact
thermometer, a software algorithm that responds to the thermometer,
a heat-transfer surface disposed within a build box, and a cooling
mechanism that operates by flowing air over a powder surface.
[0045] In another aspect, the invention features a kit for three
dimensional printing, the kit including a fluid comprising a first
solvent, a second solvent, and an absorber. The kit may also
include a first particulate adhesive material including at least
one of a thermoplastic material and a thermoset material. The first
solvent may have a boiling point above at least one of a sintering
point and a melting point of the first particulate adhesive
material.
[0046] One or more of the following features may be included. The
first solvent may include or consist of ethanol, isopropanol,
n-propanol, methanol, n-butanol, a glycol, an ester, a
glycol-ether, a ketone, an aromatic, an aliphatic, an aprotic polar
solvent, a terpene, an acrylate, a methacrylate, a vinylether, an
oxetane, an epoxy, a low molecular weight polymer, carbonate,
n-methylpyrrolidone, acetone, methyl ethyl ketone, dibasic esters,
ethyl acetate, dimethyl sulfoxide, dimethyl succinate, and
combinations thereof.
[0047] The second solvent may have a second boiling point lower
than a melting point of the first particulate adhesive
material.
[0048] The second solvent may have a second boiling point lower
than a sintering point of the first particulate adhesive material.
The second solvent may include or consist of water.
[0049] The absorber may be adapted to absorb electromagnetic
radiation at a wavelength selected from a range of 100 nm to 1 mm.
The absorber may be adapted to suscept an oscillating magnetic
field and heat by electromagnetic induction and may include or
consist of a metal, granular carbon, a polar organic compound, an
aqueous solution of an ionic substance, and a minerals having a
high conductivity.
[0050] The fluid further may include a flowrate enhancer and/or a
reactive monomer.
BRIEF DESCRIPTION OF THE FIGURES
[0051] Various features and advantages of embodiments of the
present invention will be more fully appreciated, as the same
become better understood from the following detailed description,
when considered in connection with the accompanying drawings, in
which referenced characters designate like or corresponding
parts.
[0052] FIGS. 1a-1b are schematic side views of an embodiment of an
apparatus of the present invention;
[0053] FIGS. 2a-2f and 3a-3t are schematic diagrams illustrating
processes for forming an object in accordance with embodiments of
the invention;
[0054] FIG. 4 is a schematic illustration of a circulating spreader
bead; and
[0055] FIGS. 5 and 6 are graphs illustrating the forces acting on a
particle during three dimensional printing.
DETAILED DESCRIPTION
Process Description
[0056] An embodiment of the invention features a process for
producing a three-dimensional object, including the steps of:
a) providing a layer of dry particulate material,
b) selectively applying a first absorber to one or more regions of
the particulate material,
wherein the one or more regions are selected in accordance with a
cross section of the three-dimensional object,
c) optionally causing a chemical reaction to occur between reactive
components in the particulate material, such reaction stimulated by
a liquid component otherwise serving as a vehicle for deposition of
the absorber,
[0057] d) treating the layer with electromagnetic energy that may
be spatially incoherent (i.e., unfocused and uncollimated),
polychromatic, or phase-incoherent, using an unfocused laser of
wavelength from 100 nm to 1 mm; radiant heaters or emission lamps
applying radiation comprising visible (400 nm-750 nm), IR-A (750
nm-1400 nm) or IR-B (1400-5000 nm) radiation; or by electromagnetic
induction through an oscillating magnetic field to melt, sinter, or
bond through a thermally activated chemical reaction the one or
more regions containing the first absorber to the layer of
particulate material, and, optionally, to melt, sinter, or bond
through a thermally activated chemical reaction the one or more
regions containing the first absorber with other regions located in
one or more layers situated thereunder, thereabove, or combinations
thereof,
wherein, the other regions optionally contain a second absorber,
and wherein the first absorber and the second absorber are the same
or different, and
e) cooling the layer.
[0058] In this process, the absorber is applied in accordance with
the cross section of the three-dimensional object, and specifically
may be applied in such a way that the absorber is applied only to
the regions that make up the cross section of the three-dimensional
object or to create a support structure in regions surrounding or
beneath the object.
[0059] In one embodiment of the invention, step d may be carried
after one or more consecutive executions of steps a and b. The
above method also takes into account the material-dependent
penetration depth of the electromagnetic radiation, as required by
the particulate material. For example, depending on the particulate
material and on the number of repetitions of steps a, a single
treatment with electromagnetic radiation or induction heating may
not be sufficient to melt all of the regions treated with absorber
in the layer or layers present in a construction chamber. In an
apparatus that continually applies heat into the build area, some
degree of thermal control may be required. Under instructions from
a control algorithm, the apparatus may suspend heating steps, or
modulate the exposure time during heating or modulate the cooling
time between layers.
[0060] In one embodiment of a thermal control system, a temperature
sensor, e.g., a non-contact infrared thermometer, may be used to
measure the surface temperature of the printed layers and the
information used to adjust the exposure time to the heater or the
cooling time between layers.
[0061] In another embodiment of a thermal control system, the
information derived from surface temperature measurements may be
used to modulate the concentration of absorber deposited in each
layer in order to increase or decrease the effectiveness of the
irradiation of the material.
[0062] In another embodiment of a thermal control system, the
information derived from surface temperature measurements may be
used to modulate the deposition of a quenching agent, for example,
a water-based ink not containing any absorber whose evaporation
consumes excess heat.
[0063] In another embodiment, a material may be added to the
particulate material that changes phase at an intermediate
temperature above or below the sintering or melting temperature of
the particulate material. This may create a heat sink that becomes
active at a particular temperature to reduce overheating in
irradiated areas or to provide a buffer against undesired heat
conduction into areas where sintering or melting is not
desired.
[0064] In another embodiment of a thermal control system, a
quenching agent may be applied to regions immediately outside the
volume of the part being built to cool surrounding material and
prevent it from adhering to the outside of the part. This quenching
agent does not absorb the wavelength(s) of light used to sinter the
areas printed with absorber (or absorbs them poorly). If the chosen
material has a relatively high specific heat, a high heat of
vaporization, and a boiling point below a melting point of the
polymer, the heat flowing out of the part by, e.g., through
convection, conduction, or radiation, may be dissipated by heating
and vaporizing the quenching agent material rather than melting the
particulate material. Thus, a sharp edge may be created between
sintered and unsintered areas.
[0065] A suitable quenching agent may be water, or water with
surfactants and other materials to aid the jetting process from the
inkjet. Inkjet printing inks containing additional organic
additives with moderately low boiling points may function as
quenching agents as well. 1-4 butanediol, 1-2 propanediol,
diethylene glycol, isopropyl alcohol, and/or ethyl alcohol may all
be used as quenching agents provided these substances do not
possess any solvency for the polymers in the build material. To
limit heat conduction downward, an underlying layer below the layer
to be sintered may be printed with a printhead plumbed to a
reservoir of the quenching agent. To limit heat conduction
laterally, an area around the part may be printed with quenching
agent while the absorber material is being printed. The use of
water as a quenching agent has several advantages: water is
environmentally friendly, water is compatible with inkjet
technology, and water may evaporate without leaving a residue so
that the unsintered particulate material may be reused.
[0066] In another embodiment of a thermal control system, a cooling
device may be incorporated into a surface surrounding the build
chamber, for example, in the build piston, in order to carry heat
away from the build chamber. This device may be a build plate with
an embedded cooling channel that carries a fluid heat-transfer
medium in communication with an external heat exchanger; a heat
exchanger in close thermal contact with the build plate and cooled
by airflow; or a solid-state thermoelectric (Peltier) device that
extracts heat from the build plate and conducts it into an external
heat sink. Such a mechanism may require the presence of a support
structure printed beneath the part whose density facilitates heat
conduction from the part to the cooling device across the volume of
particulate material that separates them. A support structure may
be needed for several reasons, as discussed below.
[0067] In another embodiment of a thermal control system, a cooling
device may be built onto the moving apparatus that dispenses
absorber. Such a cooling device may consist of a forced-air nozzle,
either drawing a vacuum or pushing cooled air towards the build
area.
[0068] In yet another embodiment of a thermal control system, a
mathematical model for the heat requirement may be derived from the
electronic data encoding the part; such information may be used to
program the degree of radiation exposure, absorber dosage, or any
other controlling method that is applied throughout the subsequent
build process.
[0069] For an embodiment of this process in which the absorber is
deposited in a liquid vehicle, and the liquid vehicle has a boiling
point lower than the melting or sintering temperature of the
particulate material, sufficient heat is preferably delivered to
the absorber to evaporate the liquid vehicle before bonding of
particulate material can occur.
[0070] Just enough energy to dry the particulate material prior to
sintering may be provided by passing the lamp at high speed or
lower intensity. For example, the heat typically required to boil
away a given volume of water is approximately ten times the heat
necessary to raise the same volume of most plastics to their
respective melting point. A first pre-drying pass of the heater may
allow one to preheat the material and to sinter it with lower
energy at a second pass, thus decreasing shrinkage and warping.
[0071] The step of irradiating the build area and heating the
absorber-infused portions of the build material may be postponed
until after a layer of untreated build material has been spread
over the most recently treated layer. The untreated build material
is largely transparent to the radiation, so the energy is
transmitted directly to the absorber situated immediately below.
This may cause the layer of fresh build material to be bonded very
tightly to the previous layer, enhancing the knitting between
layers. It is also quite possible that radiation treatment may not
be necessary on every layer if the radiation is capable of
penetrating absorber-treated build material to a depth greater than
one layer. After the process has been in operation for a few cycles
past the first layer, the temperature of the build may rise to a
steady-state value, reducing the overall energy requirement. An
efficient temperature-control algorithm using a non-contact
thermometer to monitor the temperature cycling may automatically
resolve whether or not any given layer requires additional
radiation.
[0072] In one particular embodiment of the present process,
radiation treatment of the build material may be performed only
after the entire layering process is complete. Certain embodiments
of the energy-application mechanism are particularly well-suited to
this process, especially microwave heating as disclosed in U.S.
Patent Publication No. 2004/0232583 A1, incorporated by reference
herein, and induction heating as disclosed herein. This step may
take place in a lower construction chamber, or in another suitable
place within the apparatus. The irradiation step may also be
carried out in an apparatus other than the apparatus used for
carrying out the spreading and printing steps, i.e., for carrying
out steps a and b, respectively. By way of example, a matrix
generated by means of steps a and b, and composed of treated layers
of particulate material, may be transferred into a commercially
available microwave oven or induction heater, where the irradiation
step is then performed. These possibilities make the present
process particularly suitable for applications in the home or
office.
[0073] Three-dimensional models may be produced by processes in
accordance with aspects of the present invention. These
three-dimensional objects, produced layer-by-layer, are present at
the end of the present process, within a matrix that is formed from
two or more layers. The object may be removed from this matrix that
is composed of fused and unfused particulate material. The unfused
particulate material may be reused, where appropriate, after
separation, for example, by sieving.
[0074] Aspects of this invention encompass articles produced by the
described process. These may be appearance models or facsimile
prototypes for design verification or pilot manufacturing of new
products. Because the mechanical properties of the thermoplastic or
thermoset materials used to form the articles are very close to
those of conventional engineering plastics, they can be used in an
extremely wide variety of applications including, but not limited
to enclosures for consumer electronics devices, mechanical
components for prototype or short-run machinery, tooling and
fixturing, and medical modeling.
Apparatus
[0075] In another embodiment, the invention features an apparatus
for the production of three-dimensional objects, comprising:
a) a means for applying a layer of dry particulate material to a
platform or to a prior layer of particulate material,
b) a means for applying one or more absorbers to one or more
selected regions of the layer of particulate material,
c) a means for generating electromagnetic energy that spatially
incoherent (i.e., unfocused and uncollimated), polychromatic, or
phase-incoherent or optionally a source for electromagnetic
induction heating,
d) optionally a means for cooling or otherwise controlling the
temperature of the regions that receive the electromagnetic or
induction energy, or are directly adjacent to such regions, and
e) optionally a computer algorithm for calculating the necessary
dosages of energy, absorber, and cooling prior to performance of
the process and adjusting these parameters during the build
process.
[0076] The present apparatus may be used for layer-by-layer
production of three-dimensional objects. The particulate build
material may be applied to an operating platform or to a previous
layer of treated or untreated build material. The means for
applying the build material and/or the absorber, include, but are
not limited to, an apparatus that moves along a plane coplanar to
the plane defined by the layer of build material, and preferably in
a vertical and/or horizontal plane. In one embodiment, the movable
apparatus consists, in part, of an operating platform. In a
preferred embodiment, the movable apparatus is present on an
operating platform, and is movable coplanar to a plane defined by
the layer of build material. In another preferred embodiment, the
movable apparatus is movable coplanar to a plane for the
application of the absorber(s) to selected regions of a layer of
build material, which defines the plane.
[0077] The absorber is preferably applied using an apparatus
movable coplanar to a plane defined by the substrate layer. The
apparatus is capable of transferring liquid and/or dry granular
absorbers at defined sites on the layer provided in step a. By way
of example, the apparatus may consist of a printing head, such as
that used in an inkjet printer. The apparatus may also contain a
guide for positioning the printing head, such as that used to guide
the printing head in an inkjet printer; the positioning of the
printing head may likewise take place in similar fashion to the
positioning of the printing head of an inkjet printer. Using such
an apparatus, the absorber is applied at those sites on the layer
provided in step a, where the substrate is to be bonded, for
example by sintering or fusion.
[0078] In an embodiment, the radiation for the described treatment
may be generated by an energy source that emits electromagnetic
radiation in the range from 100 nm to 1 mm, or by a mechanism that
supplies an oscillating magnetic field for electromagnetic
induction. Because each cross section of the three-dimensional
object is generated by the mechanism that deposits the absorber,
the radiation need not be distributed in any particular geometric
form or coherence. The form of the energy source may be spot form
or linear form or else spread form. It is also possible to combine
two or more energy sources to permit irradiation of a relatively
large area in a single step.
[0079] Introduction of energy in linear form or in spread form may
be advantageous because the selectivity is intrinsically provided
for each layer by way of the absorber or, respectively,
absorber-containing liquid applied selectively via an inkjet
process. This accelerates the process. Optionally, energy may be
delivered by electromagnetic induction by way of an oscillating
magnetic field applied to the build box. In this embodiment, energy
is applied to a thick shell encompassing the outside surface of the
part, and may penetrate entirely through certain thinner
geometries.
[0080] Referring to FIGS. 1a-1b, the present process is preferably
carried out in an inventive apparatus for the layer-by-layer
production of three-dimensional objects, which includes: [0081] a
movable component for the layer-by-layer application of a dry
particulate material on an operating platform or to a layer of a
treated or untreated particulate material which may at this stage
be present on the operating platform, [0082] a stage movable in the
x, y plane, for the application of a material including an absorber
and optionally of other additives to selected regions of the layer
composed of the particulate material, and [0083] a source of
electromagnetic energy that may be spatially incoherent (i.e.,
unfocused and uncollimated), polychromatic, or phase-incoherent,
using an unfocused laser of wavelength from 100 nm to 1 mm; radiant
heaters or emission lamps applying radiation comprising visible
(400 nm-750 nm), IR-A (750 nm-1400 nm) or IR-B (1400-5000 nm)
radiation, or a source of electromagnetic induction energy
operating in a frequency range between 5 kHz to 60 MHz, with a
preferred frequency of 13.5 MHz.
[0084] In an embodiment, a movable operating platform may also be
responsible for movements of the apparatus and, respectively, of
the energy source, and of the operating platform relative to one
another. It is also possible to use the operating platform to
realize the relative movement in the x direction and to use the
respective apparatus or, respectively, the energy source to realize
the movements in the y direction, or vice versa.
[0085] An embodiment of the present process and the present
apparatus are illustrated in FIG. 1, but there is no intention that
the invention be restricted to that embodiment. In the present
apparatus, untreated particulate build material, which has
previously been charged to a feed vessel, is built up on a movable
base 6 to give a matrix 8. A device for distributing layers of
build material, such as a doctor blade or counter-rotating
spreading mechanism 2 is used to move a portion of the build
material across the movable base and distribute a thin film of the
particulate build material over the movable base or over the
previously applied layers. The absorber 4 is applied to selected
regions of the layer composed of build material, by way of an
apparatus 3 movable in the x, y plane. After each treatment with an
absorber, a fresh layer of the build material is applied. The sites
on the applied substrate which have been treated with the absorber
are bonded by means of introduced energy of wavelength from 100 nm
to 1 mm, for example, via a heating device 5, e.g., a radiative
heater or a lamp, to give a three-dimensional object, i.e., an
article 7. This step can also take place before the application of
the subsequent layer of dry particulate build material.
[0086] Preferably, a spread layer of build material is of uniform
height. The height of the layer provided by the spreading mechanism
is preferably less than about 3 mm, more preferably from about 30
to about 2000 micrometers, and most preferably from about 80 to
about 200 micrometers. The height of each layer may determine the
resolution, and, therefore, the smoothness of the external
structure of the three-dimensional object produced. The base plate,
or else the apparatus or support for providing the layer, may be
designed with an adjustable height feature so that after the
patterning and/or subsequent heat-treatment of a given layer has
been carried out, either the resultant layer can be lowered by an
amount equal to the height of the layer to be applied next, or the
apparatus can be raised by an amount equal to the difference in
height of the next layer over the preceding layer.
[0087] Build area 22 coincides with a region 23 within which
absorber is deposited by movable stage 3, e.g., a printhead and
where the article is constructed. Three systems that operate over
this build area 22 are the printing apparatus 3 (represented here
by the printhead only, for clarity); a heating apparatus 5
(represented here as a radiant heater with reflector, by way of
example only); and the spreading roller, or "counter" roller 2 that
moves dry, free-flowing particulate build material from a source
24, across the build area 22 to form a thin layer of dry, untreated
build material prior to the printing operation. Excess build
material passes down an overflow chute 25 into a collection area
(not shown). The article 7 under construction is shown here
partially defined, with the absorber-treated regions shown in
black. This portion of the article is partly buried in untreated
build material, and is supported by a build piston 28.
[0088] In many rapid-prototyping processes, the three-dimensional
object is built up layer-by-layer. Many methods are based on the
fixation or bonding of regions of liquid layers (stereolithography)
or flowable particulate layers (laser sintering), within a layer or
among layers situated thereunder. Bonding is achieved by supplying
energy to these selected regions of the respective layers using a
focused, directed source that defines the affected regions by
imaging the energy delivery. Those regions of the layers to which
no energy is introduced remain flowable. A three-dimensional object
is obtained layer-by-layer via repetition of the particular
application and bonding or fixing of the particulate material or
liquid. Removal of the unconverted particulate material or of the
unconverted liquid gives a three-dimensional object, the resolution
of which (in relation to the outlines) depends on the layer
thickness and on the particulate grain size.
[0089] Embodiments of the present invention circumvent difficulties
encountered in the present rapid prototyping technologies in a few
ways. First, by using a non-oriented and/or non-monochromatic
and/or non-coherent energy source, a more economical energy source
can be utilized in place of amore expensive laser source with
accompanying optics. Second, the use of inkjet technology in the
preferred embodiment greatly accelerates the rate at which features
on a printed article can be defined. Whereas a laser-based rapid
prototyping system uses a single forming tool that typically
travels over the entire surface area of a given layer, an inkjet
system can use several hundred forming tools, all operating in
parallel.
[0090] In more conventional inkjet-based three dimensional
printing, exemplified by U.S. Pat. Nos. 5,204,055 and 6,007,318,
incorporated herein in their entireties, the combination of dry
particulate build material and printed fluid forms a solid article
by a direct reaction between the fluid and the particulate: in some
cases the fluid contains adhesives that particles of build
material, and in other cases, the fluid activates chemical species
contained in the build material that cause the article to solidify.
In all cases, the solvent action of the printed fluid plays a key
role in the solidification mechanism. This imposes a limitation on
the variety of materials that can be manipulated in the process
because the fluid is preferably simultaneously be compatible with
the printing apparatus and be capable of activating the
solidification of the build material.
[0091] In the present invention, the chemical nature of the fluid
is relevant only to the printing operation: it is preferably
compatible with the printing apparatus and the desired colorants or
absorbers, but it need not participate directly in the bonding of
the build material. The stimulation for bonding bond the build
material may provided by the heat developed by the absorber. While
absorbers are preferably chosen to be compatible with the fluid,
these may be engineered in the same manner as pigments, and
therefore they fall within the same province as the overall print
head compatibility that is preferred in the fluid.
[0092] In an embodiment, a process introduces energy to the build
material to be melted, sintered or otherwise bonded by way of an
absorber that absorbs the energy and transfers it in the form of
heat to the particulate build material surrounding the absorber.
The present process forms a pattern in untreated material through
the deposition of the absorber, delivered by an imaging process,
and introduces the energy from a source of radiation that need not
be focused or spatially coherent. The energy is absorbed by the
absorber, converted into heat, and transferred to contiguous build
material that is incapable of directly absorbing sufficient
radiation to bond together. In this context, the phrase "incapable
of directly absorbing sufficient radiation to bond together" means
either that the aforementioned radiation does not heat the build
material sufficiently to bond it by melting or sintering or by
thermally activated chemical reaction to adjacent particles of
build material, or that the time needed for this bonding is
excessive. By contrast, the heat transferred from the absorber is
sufficient to bond material adjacent to the absorber by melting or
sintering, or by activating a thermochemical bonding reaction and
also to melt or sinter or bond material to the absorber. The
present process can thus produce three-dimensional objects via the
melting, sintering and bonding of a granular material. The
functional principle of granular-based rapid prototyping may be
found, for example, in U.S. Pat. No. 6,136,948 and PCT Publication
No. WO 96/06881, incorporated herein by reference in their
entireties.
[0093] The absorber may be applied selectively by using
computer-controlled applications such as CAD applications used to
calculate cross sections. The absorber(s) may be applied only to
those regions of the build material within the cross section of the
three-dimensional object to be produced. A printing head apparatus
equipped with nozzles can be used for the application of the
absorber(s). Optionally, absorbers may be deposited by an
electrostatic image-transfer process similar to that used in
desktop laser printers. Once the radiation step has been concluded
for the final layer, the present process results in a matrix that
contains in part, melted, sintered or otherwise bound build
material. This matrix forms the solid three-dimensional object once
the unbound granular material has been removed.
[0094] Referring to FIG. 1b, apparatus 20 may include one or more
types of temperature controllers. For example, a non-contact
thermometer 24 may be used to monitor a temperature of the build
area 22. A software algorithm (not shown) may respond to the
thermometer 24 to control temperature-controlling methods. A
cooling mechanism 26 may flow air over a powder surface to cool
build area 22. Moreover, a heat-transfer surface 27 may be disposed
within the build box in which the build material is disposed; the
heat-transfer surface 27 may be attached to the build piston 28,
i.e., on movable base 6.
[0095] Referring to FIGS. 2a-2f, a basic cycle of an embodiment of
the inventive process is illustrated in detail, starting with the
spreading operation.
[0096] Referring to FIG. 2a, the build piston 28 is lowered,
creating space for spreading a layer of build material. A piston 30
in the build material source is raised by an amount that pushes a
preferred volume of dry, particulate build material 32 into a space
in front of the spreading, or "counter" roller 2.
[0097] Referring to FIG. 2b, the spreading roller 2 travels across
the build area 22 pushing a bead of build material 32 in front of
it, and drawing a thin layer of build material beneath, into the
space on an upper surface of the build area 22. The rotation of the
"counter" roller 2 is typically counter to the direction of rolling
along the build surface.
[0098] Referring to FIG. 2c, the printing apparatus 3 deposits a
layer of absorber 4 in regions of the build material coinciding
with a cross-section of the article to be built.
[0099] Referring to FIG. 2d, the heating device 5 is activated and
applies energy to the build area 22. A radiant heater is shown for
purposes of illustration only. Absorber-treated regions 23 of the
build material become heated, causing thermoplastic or thermoset
particulates in the layer to melt, sinter, or otherwise bond to
particles of filler that may also contained in the build
material.
[0100] Following this step, the cycle resumes as illustrated in
FIG. 2a and is repeated until the article 7 is complete, immersed
in untreated build material, and supported by the build piston 28,
as shown in FIG. 2e.
[0101] Referring to FIG. 2f, the build piston 28 is raised, and the
article 7 is removed from the untreated build material.
[0102] Referring to FIGS. 3a-3t, in an embodiment, a process cycle
incorporates the use of different build materials from different
sources for the fabrication of a three-dimensional article 7. The
components of the apparatus are similar to those discussed in the
previous illustration, with the addition of a second source 40 of
dry particulate material used as a support material 42 outside of
the regions to be formed into the article.
[0103] Referring to FIGS. 3a-3c, the particulate support material
42 is deposited onto build piston 28, and spreading roller 2
travels across the build area 22, forming a layer of support
material 42. The layer of particulate support material 42 is
sintered by the heating device 5 into a base layer 43 of bonded
material that serves as a solid substrate for melting or sintering
the build material 32 in subsequent steps. The support material 42
is chosen such that it absorbs radiation from the energy source
with no additional absorber. This permits it to fill regions where
absorber has not been deposited.
[0104] Referring to FIGS. 3d-3g, the particulate build material 32
is deposited, treated with absorber and irradiated, analogously to
the steps illustrated in FIGS. 2a-2d. In particular, a layer of
build material 32 is spread over the base layer 43 by the spreading
roller 2. The printing apparatus 3 deposits a layer of absorber 4
in regions of the build material 32 coinciding with a cross-section
of the article to be built. The heating device 5 is activated and
applies energy to the build area. Absorber-treated regions 23 of
the build material become heated, causing thermoplastic or
thermoset particulates in the layer to melt, sinter, or otherwise
bond to particles of filler that may also contained in the build
material 32.
[0105] Referring to FIG. 3h, the untreated build material 32 that
has not been made absorbing to the energy source is removed. A
vacuum nozzle 44 is shown by way of example. In this embodiment of
the invention, untreated particulate build does not become melted,
sintered or otherwise bonded when exposed to the energy source;
rather, it is the presence of the absorber in the treated regions
that bonds the build material to the solid substrate and renders
those regions substantially immune to the particulate removal
operation.
[0106] Referring to FIGS. 3i-3k, the particulate support material
42 is raised from the supply, spread over the build area 22, and
treated by the energy source 5. Because the treated regions of
build material stand out in relief in the build area 22, little or
no support material 42 is deposited in regions occupied by treated
build material. The support material 42 occupies all regions not
filled by treated build material. The effect is to create a support
structure 46 or substrate that entirely encloses the article 7 and
renews the solid substrate on the build plane over regions not
occupied by build material.
[0107] Referring to FIGS. 3l-3o, the build material 32 is spread
over the build area 22, treated and sintered in another cycle
equivalent to the cycle illustrated in FIGS. 3d-3g. In particular,
after a layer of build material 32 is spread over the base layer 43
by the spreading roller 2, the printing apparatus 3 deposits a
layer of absorber 4 in regions of the build material 32 coinciding
with a cross-section of the article to be built. The heating device
5 is activated and applies energy to the build area.
Absorber-treated regions 23 of the build material become heated,
causing thermoplastic or thermoset particulates in the layer to
melt, sinter, or otherwise bond to particles of filler that may
also contained in the build material 32.
[0108] The build material 32 covers the entire build plane 22 and
is supported everywhere by either the support material 42 or
treated build material 32 from the previous layer. Treatment with
absorber 4 occurs in another cross-section, coinciding with a slice
of the article 7 being built. When the energy source 5 is
activated, treated build material 32 melts, sinters, or otherwise
bonds to the underlying substrate. Where the treated build material
contacts the previous layer of treated build material, the treated
layer bonds to the previous layer. The build material 32 forms a
temporary bond to the support material in those regions where the
treated build material contacts the support material. This
temporary bond resists the tendency of the treated material to
contract under capillary attraction or to curl up, and it
facilitates the conduction of heat through the lower surface of the
build piston 28 by way of the continuous sintered particulate
support network physically attached to the build piston 28.
[0109] Referring to FIG. 3p, the untreated build material 32 from
this second layer is removed, e.g. by suction.
[0110] Referring to FIGS. 3q-3r, a subsequent layer of support
material 42 is spread over the second layer. In a heating step (not
shown) the support material is sintered down, in a repeat of steps
illustrated in FIGS. 3i-3k
[0111] Referring to FIG. 3s, the finished article 7 is lifted by
the build piston 28 from the build area 22. The sintered support
material 42 is shown as a solid brick or loaf entirely surrounding
the article 7.
[0112] Referring to FIG. 3t, the support material 42 is removed
from the surface of the article 7. By way of example only, the
support material is shown being dissolved by a solvent, e.g. water
or alcohol, by a spraying apparatus 48. To render embodiments of
this invention more environmentally suitable, an inexpensive
nontoxic water-soluble particulate support material such as sucrose
may be utilized in the inventive process.
[0113] In this illustration of the process, the use of two granular
materials may be regarded as an example of a more general
embodiment that utilizes several independent particulate supplies,
only one of which need be considered a "support" material. Several
independent supplies of particulate build materials, each with
different physical properties may be layered, treated, and removed
in sequence by repeating steps illustrated in FIGS. 3e-3h in series
for each independent build material to be utilized. Further, it is
not necessary to utilize every instance of build material on every
layer in the build. By this method, a composite article may be
constructed comprising different materials in different regions of
its structure.
Radiation and Energy Delivery
[0114] The means for generating the electromagnetic radiation for
the processes disclosed herein include, but are not limited to, a
source of electromagnetic energy that may be spatially incoherent
(i.e., unfocused and uncollimated), polychromatic, or
phase-incoherent, using an unfocused laser of wavelength from 100
nm to 1 mm; radiant heaters or emission lamps applying radiation
comprising visible (400 nm-750 nm), IR-A (750 nm-1400 nm) or IR-B
(1400-5000 nm) radiation, or a source of electromagnetic induction
energy operating in a frequency range between 5 kHz to 60 MHz, with
a preferred frequency of 13.5 MHz, or a chemical species present in
the build material that reacts with a printed fluid and
spontaneously generates heat by chemical reaction.
[0115] The present process has the advantage of not requiring the
use of complicated directed radiation, such as narrowly focused
laser radiation or narrowly focused microwave radiation. The
controlled exposure of certain locations of one or more layers of
build material to the electromagnetic radiation may be achieved via
the excitation of the absorber(s) by electromagnetic radiation, the
absorber(s) being applied to the desired regions of the layer or of
the layers of the build material.
[0116] The present process includes a simple way of permitting a
layer-by-layer automated build up of a three-dimensional object,
using electromagnetic radiation in combination with one or more
suitable absorbers. The build material not treated with absorber
may readily be reused, which is in contrast to processes that use
inhibitors.
[0117] Suitable types of radiative heat sources may include lasers,
especially low-cost diode lasers; incandescent lamps, especially
tungsten-halogen heat lamps, nichrome, kanthal, or silicon carbide
resistive heating elements; or high-pressure emission lamps such as
sodium-vapor or xenon. Heat sources of these types are well-known
as sources of industrial heating and are familiar to those versed
in the art. Several of these heat sources, particularly the lamps
or resistive heaters may be particularly efficient if they are
combined with reflective concentrators. Linear elements may be
provided with linear-parabolic or linear-ellipsoidal reflectors and
cylindrical lenses to concentrate the radiation. Since diode lasers
are essentially point sources of directed radiation, focusing
optics may be unnecessary if the source can be placed sufficiently
close to the patterned absorbers.
Induction Heating
[0118] Besides the direct application of spatially incoherent
(i.e., unfocused and uncollimated), polychromatic, or
phase-incoherent electromagnetic radiation, the above method also
allows the possibility of applying heat to treated materials
through electromagnetic induction. In this process, an oscillating
magnetic field is applied to the region containing the object to be
treated. This field induces an electric current in materials that
are disposed to react with that field, and the material becomes
heated through the ohmic dissipation of the induced electric
current. An absorbing substance is often referred to as a
"susceptor" in induction heating, but for purposes of consistency,
the term "absorber" is used herein for process discussions where
either induction heating or direct radiant heating may be used.
Generally, absorbers that are compatible with this process include
materials that are good electric conductors, however substances
that resonate at a particular frequency may be used so long as the
magnetic field is tuned to match.
[0119] Electromagnetic induction has been used in industry for a
very long time, particularly in the foundry industry for melting
reactive metals in an inert atmosphere. More recently,
electromagnetic induction has become incorporated into home cooking
stoves, and are currently available from manufacturers, Jenn-Air,
Kenmore, G.E., and Brandt.
[0120] The heating apparatus in an induction heater typically
consists of a coil of metal in close proximity to the volume of
application and a supply of high-frequency alternating current that
creates the oscillating magnetic field. The frequency of the field
can vary from 5 kHz to 60 MHz, with a preferred frequency of 13.5
MHz. The coil can optionally be adapted to be cooled by fabricating
it from a hollow metallic tube and passing cooling water through
it.
[0121] Like microwave heating, energy is absorbed in a thick shell
surrounding the outside surface of the part. Both microwave and
induction heating may be subject to lack of temperature control in
parts that have large variation in section thickness. PCT
Publication WO 2004/048463 A1, incorporated herein by reference,
includes an extensive discussion of modes by which microwave
heating, and by extension, induction heating, may produce
non-uniform heating in freeform plastic parts. Energy is absorbed
in bulk regions, and less so in thin sections. Additionally,
cooling occurs at the parts surface, so the temperature tends to be
lowest on the outside surface of the part. For these reasons,
microwave and induction heating are somewhat less preferred than
direct infrared heating of many thin layers in sequence, as
described herein.
Chemical Sintering
[0122] In an embodiment, the invention includes an alternative to
externally applied energy. Instead, chemical energy is derived
reactive components in the printed liquid and/or in the particulate
build material. If one of the components of the build material
releases heat when it is contacted by the printed liquid, i.e., by
an exothermal dissolution of an anhydrous ingredient on contact
with a water-based printed fluid, the increased temperature in the
immediate vicinity of the fluid pattern may promote melting and/or
sintering of other components present in the build material.
Another alternative includes a combination of two reactants in the
build material, whose exothermal reaction is initiated or supported
by the presence of the printed fluid. An example of two reactive
particulate materials is a combination of an acid and an alkali.
Many chemicals in these categories are available as free-flowing
particulates, and they may be combined in the dry state without any
significant reaction occurring until moisture is added to bring the
reactants together. On the application of liquid water, one of the
ingredients (the acid, for example) may dissolve, and react with
the other ingredient (that need not dissolve: the alkali, in this
example) to form a salt by their combination, releasing chemical
energy and causing the materials in the vicinity to warm up. The
heat supplied by these mechanisms may be sufficient to melt,
sinter, or otherwise bond by a thermally activated chemical
reaction, structural precursors also present in the build material.
This process differs slightly from the previously mentioned process
because the heat required for bonding is supplied spontaneously and
locally, rather than being directed by a broadly applied external
source.
[0123] It may be advantageous to heat the layers to be sintered to
an elevated temperature, via introduction of bulk heating of the
build chamber. It may also be advantageous to keep the layers at an
elevated temperature, this temperature being below the melting or
sintering point of the polymer used. This method can reduce the
amount of electromagnetic or chemical energy needed for the melting
or sintering process. A precondition for this is the presence of a
temperature-controlled construction space that also reduces the
likelihood of curl-up of the corners and edges of the patterned
layers that can make it difficult to spread a smooth layer of loose
build material over previously printed regions. It may also be
advantageous for the absorber or the absorber-containing liquid to
be preheated.
Cooling and Energy Dissipation
[0124] To better control the application and disposal of energy in
the build process, a cooling device may be incorporated into a
surface surrounding the build chamber, for example, in the build
piston, to carry heat away from the build chamber. This device may
be a build plate with an embedded cooling channel that carries a
fluid heat-transfer medium in communication with an external heat
exchanger; a heat exchanger in close thermal contact with the build
plate and cooled by airflow; or a solid-state thermoelectric
(Peltier) device that extracts heat from the build plate and
conducts it into an external heat sink. This device may alleviate
the buildup of heat in the deeper portions of the build that may
have already become sufficiently well bonded, but still contain
absorber and can still potentially become heated by the energy
source.
[0125] Such a mechanism may require the presence of a support
structure in the build area beneath the part whose structure
facilitates heat conduction from the part to the cooling device
across the volume of particulate material that separates them.
[0126] In another embodiment of a thermal control system, a cooling
device may be built onto the moving apparatus that dispenses the
absorber. Such a cooling device may consist of a forced-air nozzle,
either drawing a vacuum or pushing cooled air towards the build
area.
[0127] In another embodiment of a thermal control system, a
substance may be incorporated into the particulate build material
that changes in phase (e.g., by melting or evaporation) at a
temperature that is particularly beneficial to the process. In one
embodiment, this may be a temperature slightly higher than the
sintering temperature of a thermoplastic component in the build
material. In this embodiment, the phase change material prevents
the temperature from rising too quickly above the required
temperature and prevents overheating of the build material during
irradiation. In another embodiment, the phase change occurs at a
temperature somewhat below the sintering temperature of the build
material. This retards the bonding of the build material until a
certain threshold dosage of radiation has been absorbed. The method
may be used to prevent material adjacent to absorber-treated
regions from sintering due to heat conduction away from the
absorber-treated regions.
[0128] Methods for Forming Layer Patterns of Absorber on the
Substrate
[0129] The method by which an absorber is deposited on the surface
of the build material may vary, but a preferred method is to
deposit it by inkjet printing of absorber in a liquid carrier. In
this embodiment, the process represents an improvement over an
early three-dimensional printing technique described in U.S. Pat.
No. 5,204,055. That reference describes the use of an inkjet style
printing head to deliver a liquid or colloidal binder material to
sequentially applied layers of dry particulate material. The
three-dimensional inkjet printing technique or liquid binder method
involves applying a layer of a particulate material to a build
surface using a counter-roller. After the particulate build
material is applied to the build area, the inkjet printhead
delivers a liquid binder in a predetermined pattern to the layer of
build material. The binder infiltrates into gaps between grains in
the build material and hardens to bond the build material into a
solidified layer. In subsequent improvements, certain components
are incorporated into the build material that participate in
chemical reactions activated by the liquid binder, the binder
serving more as a reaction medium than an actual adhesive in
itself. These improvements are disclosed in U.S. Pat. No.
5,902,441, U.S. Pat. No. 6,610,429, European Patent No. EP 1226019
B1, U.S. Patent Publication No. 2004/0056378 A1, and U.S. Patent
Publication No. 2005/0003189; all of these references are
incorporated herein by reference in their entireties. In
embodiments of the present invention, the liquid binder acts as an
absorber or a carrier for the absorber, and the primary mode of
solidification is by the action of heat transferred from the
absorber to adjacent grains of particulate build material.
Additionally, the liquid carrier may exhibit some functionality as
a "binder," i.e., some solvent or chemical activity towards
components in the build material, and facilitate hardening by a
secondary bonding mechanism such as dissolution of soluble
polymeric adhesives. The bound build material also bonds each layer
to the previous layer. After the first cross-sectional portion is
formed, the previous steps are repeated, building successive
cross-sectional portions until the final article is formed.
[0130] As used herein, the term "build surface" refers to the
exposed surface, usually flat, planar and facing upwards, of the
volume within which three-dimensional parts are built in a 3D
Printer. This surface coincides with the plane of spreading of the
particulate build material, and it coincides with the substrate
plane upon which patterns of absorber are deposited. In embodiments
of the invention, the mechanism that deposits the absorber travels
mostly in a plane parallel to the build surface, displaced a short
distance vertically, with optional small relative movement along a
line that connects the two surfaces. This relative movement may be
caused by motion of the deposition mechanism towards the build
surface, or by motion of the platform that supports the build
surface in the direction towards the deposition mechanism.
[0131] As used herein, the term "counter-roller" refers to a
particularly preferred mechanism for spreading a thin film of
particulate build material over a surface. In an embodiment of the
invention, the surface is the "build surface" of a 3D Printer. The
mechanism acts by pushing a bead of free-flowing dry particulate
build material in front of a cylindrical roller (the
counter-roller) that rotates counter to the direction of its
motion. The advancing surface of the roller tends to lift unused
build material and cause it to tumble in a wave that is pushed
along by the roller. This method provides a relatively smooth, thin
layer of build material across a wide range of mechanical
properties of build materials. These mechanical properties are
discussed elsewhere in this document. The use of a counter-roller
is quite well established in freeform fabrication; examples of its
use are described in U.S. Pat. No. 5,053,090, incorporated herein
by reference in its entirety, as well as in U.S. Pat. No.
5,204,055.
[0132] As used herein, the term "binder" refers to a fluid
component that is deposited by one of the various methods described
in the various embodiments that either possesses an adhesive
component in solution or suspension; or it is capable of activating
an adhesion or some other solidification phenomenon by virtue of
its solvent properties or its chemical nature. This is
distinguished from the term "carrier" which is used herein to
describe a fluid component that is deposited by one of the various
methods in the various embodiments that does not possess the
capacity, in itself, to cause any adhesion between grains of
particulate build material in the build area. A carrier may be used
to deliver an absorber to the build material, either in suspension
of in solution, with the absorber supplying the heat necessary to
cause bonding between grains of build material.
[0133] In another embodiment of the invention, the absorber may be
deposited in a liquid slurry under steady pressure through a nozzle
that is translated over the build surface. This is similar to an
extrusion process that is not necessarily capable of the same
switching speed or resolution as an inkjet printhead. In a
preferred embodiment, there is one nozzle per species of absorber,
and with each nozzle being translated in a plane parallel to the
build surface in a path that conforms to the contours of the layer.
This is distinct from the motion of a multiple-nozzle inkjet
printhead, which is most preferably passed over the build surface
in a raster pattern. In this embodiment, an absorber fluid is a
very viscous, highly loaded liquid or gel that may be desirable in
some applications. These may include the fabrication of ceramic or
metal parts in which a specialized absorber is preferably
compatible with high processing temperatures and contribute a
significant fraction of solids to avoid porosity in the completed
part, or if a preferred material cannot be milled to a fine enough
particle size (below 1 .mu.m) to be suspended in a carrier for
inkjet printing. Such materials include reactive metals such as
aluminum, magnesium, and titanium.
[0134] In another embodiment of this invention, the absorber is
deposited electrostatically by means of a photoconductive plate as
used in conventional laser printers. A similar process has been
disclosed in U.S. Pat. No. 6,531,086, incorporated herein by
reference in its entirety. In the aforementioned patent, an opaque
mask is created using an electro-photographic process, and the
build surface is irradiated through the mask. The radiation is
projected into an image of a cross-section of a desired model, and
the particulate build material strongly absorbs the radiation. This
approach typically requires the use of a collimated (spatially
coherent) radiation source, unlike embodiments of the present
invention.
[0135] In the first step of the present embodiment of the instant
invention, a photoconductive plate (or cylinder) is charged by an
electrostatic discharge. The image of a cross-section of a layer is
projected onto the surface of the charged photoconductor by an
optical system, or it is written digitally by a switched laser
beam. Points where the light strikes the photoconductor become
neutralized, creating a "latent" image of charged areas on the
plate or drum. Dry particulate absorber is then dusted onto the
surface of the plate. The absorber is formulated such that it
adheres to the charged surface of the plate, but falls off the
uncharged surface. A layer of loose build material is spread over
the build surface and the photoconductive plate is translated over
the build surface and fully discharged, causing the
electrostatically held particles of absorber to detach from the
plate and fall onto the build area. The photoconductive plate is
removed from the build area and the absorber is irradiated by an
unfocused, spatially incoherent source of electromagnetic
radiation, causing the build material to melt or sinter to form a
solid layer. A second layer of build material is spread over the
build area to form a substrate for the next layer of absorber, and
the photoconductor is re-charged to receive the image of the next
layer.
[0136] In still another embodiment of this invention, free-flowing
particulate build material is spread onto the build surface and the
absorber is deposited onto the build surface through a stencil in
an aerosol or otherwise by spraying, or squeezed through a
silkscreen as a paste or gel. Each different pattern for a layer is
typically fabricated separately as a different stencil or screen.
While such a process may not be economically feasible for small
runs of freeform parts, it may become very economical for
large-scale production of freeform parts or short-run production of
parts with simple geometries. A process analogous to the liquid
binder process disclosed in U.S. Pat. No. 5,204,055, but using
stencils, is described in U.S. Pat. No. 5,940,674. In the present
embodiment, the pattern is formed by application of an absorber,
rather than by application of an adhesive binder.
Liquid Carrier or Binder
[0137] In another aspect, the invention features a fluid for
three-dimensional printing, the fluid including a first solvent
having a first boiling point, and a second solvent having a second
boiling point. The fluid may include water. The first solvent may
be water-miscible. The second solvent may be water-miscible. The
second solvent may have a second boiling point that is higher than
the first boiling point. The fluid may also include a surfactant, a
rheology modifier, and/or an amine. The fluid is adapted to carry
an absorber material, either in suspension or in solution, or it
may be an absorber in itself. Further, the fluid may be adapted to
activate an adhesive in a particulate build material comprising a
blend of a thermoplastic or thermoset particulate material in
combination with an adhesive particulate material, or may
participate in a chemical reaction with reactive components in the
build material to facilitate hardening of the structure.
[0138] Many aspects of the fluid have already been disclosed in a
previous application, in particular U.S. Patent Publication No.
2005/0003189, incorporated herein by reference in its entirety. The
inventive aspect of the fluid disclosed herein is in the adaptation
for carrying an absorber material into the printed regions of the
build material, and subsequently irradiating or applying
electromagnetic induction to the build area to stimulate either
melting or sintering of thermoplastic or thermoset components
contained in the build material.
Absorber Materials
[0139] Absorbers (first absorber and/or second absorber) that may
be used in the present process are any of those which are heated by
electromagnetic radiation of wavelength from 100 nm to 1 mm. In an
embodiment, the absorbers are any that are heated by
electromagnetic induction in a frequency range between 5 kHz to 60
MHz, with a preferred frequency of 13.5 MHz.
[0140] In the simplest case, the absorber comprises or consists
essentially of a colorant. A colorant is defined as any substance
that imparts color to another material or mixture, being divisible
into inorganic and organic colorants, and also into natural and
synthetic colorants (see Hawley's Condensed Chemical Dictionary,
14.sup.th Ed. (2001) p. 287, incorporated herein by reference. A
pigment is an inorganic or organic colorant whose color is
non-neutral or neutral and which is practically insoluble in the
medium in which it is used. Dyes are inorganic or organic colorants
whose color is non-neutral or neutral and which are soluble in
solvents and/or in binders.
[0141] However, the absorbent action of an absorber may increased
by including additives. For example, additives may be flame
retardants based on melamine cyanurate (MELAPUR from DSM) or based
on phosphorus, preferably phosphates, phosphites, phosphonites or
elemental red phosphorus.
[0142] An absorber system disclosed in WO 2004/048463 A1 is
specifically directed to thermoplastic polyolefins, and is
compatible for use in the instant process. This absorber system
includes a combination of a metallic pigment in combination with a
tertiary amine or phosphine. At least one of these components is
typically adapted to inkjet printing or another of the
above-referenced deposition processes, while the other component
may be blended directly into the particulate build material. While
recent developments have resulted in metallic pigments in inkjet
inks, it is more likely that the component to be deposited may be
the amine or phosphine, since these are generally soluble in some
solvent that can be used as a carrier.
[0143] The absorber present in the build material preferably
includes a principal component of carbon black or copper hydroxide
phosphate (CHP), or chalk, animal charcoal, carbon fibers,
graphite, flame retardants, or interference pigments.
[0144] CHP may be used in the form of a pale green, fine
crystalline particulate material whose median grain diameter is
just 3 .mu.m. Suitable CHP may be, for example, VESTODUR FP-LAS
from Degussa.
[0145] The carbon black may be prepared by the furnace black
process, the gas black process, or the flame black process,
preferably by the furnace black process. The primary particle size
is from 10 to 100 nm, preferably from 20 to 60 nm, and the grain
size distribution may be narrow or broad. The BET surface area, in
accordance with DIN 53601, is from 10 to 600 m.sup.2/g, preferably
from 70 to 400 m.sup.2/g. The carbon black particles may have been
subjected to oxidative post-treatment to obtain surface
functionalities. They may be hydrophobic (for example Printex 55 or
flame black 101 from Degussa) or hydrophilic (for example FW20
carbon black pigment or Printex150 T from Degussa). Other examples
of carbon black are Printex 60, Printex A, Printex XE2, and Printex
Alpha from Degussa. They may have a high or low level of
structuring, i.e., the degree of aggregation of the primary
particles. Specific conductive carbon blacks can be used to adjust
the electrical conductivity of the components produced from the
inventive build material. Better dispersibility in both the wet and
the dry mixing processes may be obtained by utilized using carbon
black in bead form. It may also be advantageous to use carbon black
dispersions.
[0146] Animal charcoal is an inorganic black pigment comprising
elemental carbon. It is composed of from 70 to 90% of calcium
phosphate and from 30 to 10% of carbon. Density is typically from
2.3 to 2.8 g/ml.
[0147] Interference pigments are also referred to as pearlescent
pigments. Using the natural mineral mica as a basis, they are
encapsulated with a thin layer composed of metal oxides, such as
titanium dioxide and/or iron oxide, and are available with a median
grain size distribution of from 1 to 60 .mu.m. By way of example,
interference pigments are supplied by Merck with the name Iriodin.
The Iriodin product line encompasses pearlescent pigments and
metal-oxide-coated mica pigments, and also the subclasses of:
interference pigments, metallic-cluster special-effect pigments
(iron oxide coating on the mica core), silvery white special-effect
pigments, gold-luster special-effect pigments (mica core coated
with titanium dioxide and with iron oxide). The use of Iriodin
grades in the Iriodin LS series is particularly preferred, namely
Iriodin LS 820, Iriodin LS 825, Iriodin LS 830, Iriodin LS 835, and
Iriodin LS 850. The use of Iriodin LS 820 and Iriodin LS 825 is
most particularly preferable.
[0148] Other suitable materials for use as pigments are: mica or
mica pigments, titanium dioxide, kaolin, organic and inorganic
color pigments, antimony (III) oxide, metal pigments, pigments
based on bismuth oxycholoride (e.g. the Biflair series from Merck,
high-luster pigment), indium tin oxide (nano-ITO powder from
Nanogate Technologies GmbH or AdNano.TM. ITO from Degussa),
AdNano.TM. zinc oxide (Degussa), lanthanum hexachloride,
ClearWeld.RTM. (disclosed in WO 0238677), and also commercially
available flame retardants that include melamine cyanurate or
include phosphorus, preferably including phosphates, phosphates,
phosphonites, or elemental (red) phosphorus.
[0149] Many of the pigments mentioned in the preceding paragraphs
are not available for inkjet printing formulations. Accordingly,
their use may be restricted to one of the other embodiments listed
above that do not require a finely dispersed colloid or a solution
in a carrier liquid. These may include, for example, the slurry
deposition process or the dry electrophotographic process.
[0150] If the intention is to avoid any adverse effect on the
intrinsic color of the model, the absorber preferably comprises
interference pigments, particularly preferably from the Iriodin LS
series from Merck, or Clearweld.RTM..
[0151] The absorbers may, by way of example, be in pellet form,
particulate form, or liquid form. For distribution within a
printing head with one or more fine nozzles it is advantageous for
the particles to be especially fine, and therefore excessively
coarse particles or pellets may be milled or further milled,
preferably at low temperatures, and then optionally classified.
[0152] Absorbers include, but are not limited to, particulate
substances, e.g., metal powders, metal compounds, ceramic powders,
graphite, carbon black, or activated charcoal. In certain
embodiments, the fluid deposited on the build surface might
constitute the absorber. The deposited fluid may be water or protic
liquids such as saturated mono- or polyhydric linear, branched, or
cyclic aliphatic alcohols, or mixtures thereof, each undiluted, or
mixed with water. Preferred protic liquids include glycerol,
trimethylolpropane, ethylene glycol, diethylene glycol, butanediol,
or mixtures thereof, each undiluted, or mixed with water. Other
examples include polar organic compounds such as amines,
phosphines, glycols, polyglycols, and polyelectrolyes. It is also
possible to use a mixture of absorbers, containing one or more
liquid absorbers, one or more solid absorbers, or combinations of
liquid and solid absorbers. It may also be advantageous to suspend
solid absorbers in liquid carriers that are not absorbers, in order
to achieve better distribution of the solid absorbers over the
entire depth of the substrate layer provided. The absorber, in
particular a liquid absorber, may also be equipped with surfactants
for better wetting of the substrate. The choice of a liquid
absorber depends upon the absorption characteristics of the liquid
as compared to the spectrum of radiation projected by the source.
For example, water absorbs infrared light very strongly in a broad
range of wavelengths starting from a minimum around 1000 nm.
Absorbers for Induction Heating
[0153] In one embodiment of this invention, energy is applied to
the build material as a high-frequency oscillating magnetic field,
and absorbers are materials that react to this magnetic field such
that they are heated through electromagnetic induction. An
absorber, as used herein, denotes an ingredient that heats
sufficiently to melt or sinter or activate a chemical bonding
reaction between the structural components of the build material
when exposed to electromagnetic radiation or electromagnetic
induction. Absorbers that operate through induction are generally
substances with a relatively high electrical conductivity, or they
develop a high conductivity when they are dissolved in the liquid
carrier.
[0154] The major classes of absorbers include metals, present
either in particulate form or as coatings on inorganic
particulates; granular carbon; polar organic compounds, including
polymers and non-polymers; aqueous solutions of ionic substances,
especially salts that impart a high electrolytic conductivity to
the solution; and certain minerals with high conductivity,
including minerals that are semiconductors and minerals that are
ionic conductors. The various classes of absorbers are disclosed in
WO 2004/048463 A1, incorporated herein by reference.
[0155] Metal absorbers include all of the representatives of the
class: irons and steels, copper, brasses, bronzes, aluminum, zinc,
tin, lead, solder, silver, gold, and so forth. For preferred
embodiments that utilize inkjet printing, nano-disperse metal
particles may be suspended in a liquid carrier and filtered to a
grain size below 1 .mu.m to be compatible with the printing
mechanism. While there are few limitations to doing this in
principle, representatives that are currently commercially
available are limited to silver and gold (available from Cabot
Corp.) The principal limitation to incorporating other metal
particles into inkjet suspensions is the rate of oxidative
corrosion of finely dispersed metals: if reactive metals such as
aluminum, magnesium and titanium are milled to grain sizes below
about 10 .mu.m, they become highly reactive, even pyrophoric, and
so may be unsuitable for inkjet printing. These materials might be
useable in one of the other embodiments, particularly the slurry
technique or the electropohtographic technique described above.
[0156] Carbon black is a preferred absorber for embodiments using
electromagnetic radiation in the visible and IR ranges as well. It
possesses a sufficiently high electrical conductivity to be used as
an absorber for electromagnetic induction as well as for microwave
absorption. Inkjet printing inks containing relatively high volume
fractions of carbon are commercially available as black printing
inks, and may be used up to about 20% solids by volume. A
commercial product used in the examples given below is
Cab-O-Jet-200 Black pigment, manufactured by Cabot Corp. of
Haverhill, Mass.
[0157] Polar organic compounds that may be used as absorbers may
include or consist or amines, phosphines, glycols, organic acids,
polyglycols, and polyelectrolyes. These are substances that possess
a high degree of electric polarizability and may react strongly to
particular frequencies of electromagnetic radiation. Many of these
substances are soluble in water or other solvents compatible with
inkjet printing.
[0158] Examples of mineral absorbers that are semiconductors are
zinc oxide and reduced iron oxide, FeO. Several minerals are ionic
conductors, and their use has been reported as absorbers for
microwave and induction heating. These include zeolites,
bentonites, acid phosphate salts such as monopotassium phosphate.
Other metal-organic materials such as titanium and zirconium
hydroxyethyl phosphonate have been reported.
[0159] The absorber may be an aqueous solution of an ionic
substance, such as phosphoric acid, hydrochloric acid, zinc
chloride, stannous chloride, lithium perchlorate, or lithium
acetate. Almost any soluble salt, acid or alkali might be chosen,
although those listed above are preferred by virtue of their very
high electrolytic conductivity in aqueous solution. Suitable
absorbers are described in, for example, U.S. Pat. Nos. 6,600,142
and 6,348,679, incorporated by reference herein in their
entireties. Some salts may be incorporated into the build material
as dry particles. These may have very low electrical conductivity
in their dry state, but become active as absorbers when moistened
by the fluid component deposited by the printing mechanism. In some
embodiments, the absorber may be soluble or even dispersible in
non-aqueous solvents to be utilized in induction heating.
Heat-Generating Materials for Chemical Sintering
[0160] In an embodiment, a substance is incorporated into the build
material that spontaneously releases thermal energy when combined
with the fluid component printed in a pattern on the build surface.
These include ionic substances that dissolve exothermally in
aqueous printing fluids; combinations of substances that react with
one another when activated by the printing fluid, and substances
that react chemically with the printing fluid.
[0161] Examples of exothermic ionic substances include calcium
chloride, anhydrous magnesium sulfate, trisodium phosphate, and
sodium and potassium hydroxides. Reacting systems include alkaline
oxides in combination with dry organic acids.
[0162] Examples of suitable alkalis include calcium, zinc, or
magnesium oxides, sodium silicate, sodium or potassium hydroxides,
trisoduim phosphate. These may be mixed in any combination with dry
particulates of citric acid, tartaric acid, succinic acid, adipic
acid, malic acid, malonic acid, maleic acid, glycolic acid,
glutaric acid, or anhydrides thereof.
[0163] The alkalis in the previous example may be caused to react
with acidic solutions or liquid acid anhydrides such as glacial
acetic acid, aqueous phosphoric, sulfuric, nitric or hydrochloric
acid, anhydrous lactic acid, and acidic solutions of salts such as
monocalcium phosphate, mono-ammonium phosphate, aluminum acid
phosphate, zinc or magnesium chlorides, or mixtures thereof.
Build Materials
[0164] Build materials that are suitable for the processes
described herein include thermoplastic particulate materials, inert
fillers coated with thermoplastic materials, thermoset materials,
inert fillers coated with thermoset materials, combinations
thereof, and combined with more conventional solvent-activated
adhesives and reactive fillers as described in European patent EP
1226019 B1, and U.S. Patent Publication Numbers US 2001/0197431 A1,
US 2004/0056378 A1, and US 2005/0003189 A1, incorporated herein in
their entireties.
[0165] As used herein, "thermoplastic particulate material" is
meant to define a particulate material that becomes bonded when the
adhesive particulate material is activated by a fluid, the
component including a material that may be repeatedly softened by
heating and hardened again on cooling.
[0166] As used herein "thermoset particulate material" is meant to
define a class of materials that include a continuous thermoplastic
phase, but also include a segment that chemically crosslinks during
thermal processing. Thermoset materials have the property of
becoming permanently hard and rigid when heated or cured. Upon
heating, thermoset materials undergo chemical crosslinking, thereby
increasing the molecular weight of the polymer chain.
Thermoplastic Particulate Materials
[0167] At least a part of the particulate material used may be
amorphous, crystalline, or semicrystalline. A preferred particulate
material has a linear or branched structure. Particularly preferred
particulate material has, at least in part, a melting point of from
about 50 to about 350 degrees C., preferably from about 70 to about
200 degrees C.
[0168] Particulate materials suitable in the present process are
substances whose susceptibility to heating by, or absorption of,
electromagnetic radiation of wavelength from 100 nm to 1 mm that of
the selected absorbers. The particulate materials preferably also
exhibit sufficient flowability in the heated state. Preferred
materials have a melt flow index at 230.degree. C. of at least 2.0
g/10 min; preferably higher than 10 g/10 min, and most preferably
about 20 g/10 min. Particulate materials that may be used include
polymers or copolymers, including, but not limited to, polyester,
polyvinyl chloride, polyacetal, polypropylene, polyethylene,
polystyrene, polycarbonate, polybutylene terephthalate,
polyethylene terephthalate, polysulfone, polyarylene ether,
polyurethane, polylactides, thermoplastic elastomers,
polyoxyalkylenes, poly(N-methylmethacrylimide) (PMMI), polymethyl
methacrylate (PMMA), ionomer, polyamides, copolyester,
copolyamides, silicone polymers, terpolymers,
acrylonitrile-butadiene-styrene copolymers (ABS), polyether
sulfone, polyaryl sulfone, polyphenylene sulfide, polyaryl ether
ketone, polyphthalamide, polyimide, polytetrafluoroethylene, or
mixtures thereof.
[0169] The thermoplastic particulate material may include at least
one of polyphenylsulfone, polyacrylonitrile, polycondensates of
urea-formaldehyde, polyolefins, cyclic polyolefins, polyvinyl
butyral, polyvinyl chlorides, acrylics, ethyl cellulose,
hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose,
cellulose acetate, hydroxypropylmethyl cellulose,
hydroxybutylmethyl cellulose, hydroxyethylmethyl cellulose,
ethylhydroxyethyl cellulose, cellulose xanthate, and combinations
and copolymers. Preferred thermoplastic materials are amorphous
thermoplastics with a high melt-flow index. The build material
preferably includes amorphous cyclic polyolefin polymers such as
ZEONOR.RTM. (Zeon Chemicals), TOPAS.RTM. (Ticaona), polyacrylates
and polymethacrylates and their copolymers such as Plexiglas (Rohm
& Haas), LUCRYL.RTM. (BASF), LUCITE.RTM. (Dupont), polymethyl
methacrylamide such as KAMAX (Rohm & Haas); polystyrene such as
STYRON.RTM. (Dow Chemicals), metallocene grade polyolefins such as
ACHIEVE.TM. and EXCEED.TM. (ExxonMobil).
Thermoset Particulate Materials
[0170] Thermoset compositions are also suitable for use as build
materials. A thermosetting composition, as used herein, refers to
any single-component or multi-component reactive system that can
crosslink by poly-condensation and/or radical and/or by a more
specific polymerization route, passing from a state of a liquid or
paste or solid with a soluble and/or fusible structure to the state
of a solid with an infusible and insoluble structure. These
materials have the property of becoming permanently hard and rigid
when heated or cured. Upon heating, thermoset materials undergo
chemical crosslinking, thereby increasing the molecular weight of
the polymer chain. Examples of suitable thermoset compositions
include epoxy with aromatic and aliphatic amines, amides, acid
anhydrides, and multifunctional acids; isocyanate/amine,
isocyanate/alcohol, unsaturated polyesters, vinyl esters,
unsaturated polyester and vinyl ester blends, unsaturated
polyester/urethane hybrid resins, polyurethane/urea, reactive
dicyclopentadiene resin, reactive polyamides, or polyester
sulfones. These materials are sold under brand names such as
Transparent.RTM., Everclear.RTM., and Nap-Gard.RTM., manufactured
by DuPont; E-, H-, and U-series powder-coating materials
manufactured by Prizm Powder Coatings; and Crelan.RTM. manufactured
by Bayer. Another example of a suitable thermoset composition is a
moisture-curable hot melt polyurethane, such as Jet-Weld.RTM. from
3M.
[0171] Other examples include particulate thermoset materials such
as pulverized/encapsulated epoxy and pulverized dicyanamide that
react together. Also, high molecular-weight polyols, polyamines,
and polythiols may be combined with isocyanates, diacids,
polyacids, and multifunctional acid anhydrides such that they may
react very slowly at ambient temperature, but may react and
solidify when heated.
[0172] In an embodiment, the build material includes thermoset
compositions and/or thermoplastic vulcanizate (TPV). A TPV is a
class of materials that include a continuous thermoplastic phase,
but also include a segment that chemically crosslinks during
thermal processing. This crosslinking reaction is irreversible once
it has occurred. TPVs' mechanical and elastic recovery properties
may be superior in comparison to these respective properties of
thermoplastic elastomers. Examples of TPVs include ethylene
propylene diene monomer (EPDM) and/or hydrogenated styrene block
copolymer (HSBC) dispersed in a polypropylene matrix sold under the
brand names of Santoprene.RTM. and Uniprene.RTM.. Other suitable
high-performance TPVs are based on nylon/polyacrylate,
nylon/silicone, and copolyester/elastomer formulations. By way of
example, Zeotherm 100 Series produced by Zeon chemicals is based on
polyacrylate (ACM) elastomers dispersed in a polyamide (nylon)
plastic matrix; TPSiV.TM. is a Thermoplastic Silicone Vulcanizate
produced by Multibase which is the division of Dow Corning;
Nexprene.RTM. by Solvay is based on nitrile rubber dispersed in the
polyolefin matrix; EPTV is a material developed by DuPont combining
a copolyester matrix material with a highly cross-linked rubber
modified ethylene-acrylate as the vulcanized segment.
[0173] In another embodiment, the absorber ink is applied via an
inkjet printhead onto a layer of particulate build material in
areas where the two-dimensional cross-section of the
three-dimensional article is to be formed. Through a separate
printhead, another fluid that is reactive to form a thermoset
material is applied over the same regions where the absorber was
printed. The printed area of each successive layer is exposed to
induction where the absorber is heated to a temperature that
initiates the cure of the second fluid to form a thermoset material
and solidifying the region. The regions of the build material where
the absorber was not applied remain cool and flowable, and may be
reused again. Some examples of thermoset resins include: inkjetable
epoxy with a viscosity low enough to be processed by a conventional
drop-on-demand printhead, (generally below 20 mPa-s) printed onto a
particulate build material containing an acid catalyst to initiate
a cationic polymerization, or over pulverized dicyandiamide, adipic
dihydrazide, or succinic dihydrazide, to melt and react with the
epoxy when heated; inkjet-able one-component epoxy with latent
amine curatives or latent acid catalysts that react only when
heated; and inkjet-able acrylate (and/or methacrylate) monomers and
oligomers with a peroxide catalyst included in the build material
and that reacts only when heated.
Fillers
[0174] Other additives may be incorporated into the particulate
build materials of the invention, such as inert fillers. These
fillers help to reduce shrinkage of the three-dimensional object
because they retain their shape to a substantial extent during the
radiation treatment. In addition, the use of fillers permits, by
way of example, alteration of the plastic and physical properties
of the objects. In one preferred embodiment, the inventive build
material contains from about 1% to about 70% by weight, preferably
from about 5% to about 50% by weight, and more preferably from
about 10% to about 40% by weight, of fillers, based on the total
weight of the build material and having a mean particle diameter of
about 5 micrometers to about 100 micrometers.
[0175] The inert filler material may include an inorganic material
chosen such that it is either transparent or highly reflective to
the radiation used to heat the absorber. Such filler material may
include or consist essentially of soda-lime glass, borosilicate
glass, aluminosilicate ceramic, limestone, plaster, bentonite,
precipitated sodium silicate, amorphous precipitated silica,
amorphous precipitated sodium silicate, amorphous precipitated
lithium silicate, salt, aluminum hydroxide, magnesium hydroxide,
calcium phosphate, sand, wollastonite, dolomite, amorphous
precipitated silicates containing at least two types of ions
selected from sodium ions, lithium ions, magnesium ions, and
calcium ions, metallic oxides such as titanium dioxide, aluminum
oxide, magnesium oxide, zinc oxide, silica (amorphous, fumed, or
crystalline), calcium carbonate, magnesium carbonate, gypsum, talc,
clay, boron nitride, olivine, calcium silicate, magnesium silicate,
amino-silane surface-treated soda lime glass, epoxy-silane treated
soda-lime glass, amino-silane treated borosilicate glass,
epoxy-silane treated borosilicate glass, and amino-silane surface
treated calcium silicate.
[0176] The inert filler material may include an organic material.
The organic material may include or consist essentially of a
carbohydrate, such as starch, modified starch, cellulose,
maltodextrin, acacia gum, locust bean gum, pregelatinized starch,
acid-modified starch, hydrolyzed starch, sodium carboxymethyl
cellulose, sodium alginate, hydroxypropyl cellulose, methyl
cellulose, chitosan, carrageenan, pectin, agar, gellan gum, gum
Arabic, xanthan gum, propylene glycol alginate, guar gum, gelatin,
rabbit-skin glue, soy protein, gluten, and combinations
thereof.
[0177] The particulate build material may include an inert
reinforcing fiber. The reinforcing fiber may include at least one
of the following materials: natural polymers, modified natural
polymers, synthetic polymers, ceramic, fiberglass, polyamide flock,
cellulose, rayon, polyvinyl alcohol, and combinations thereof.
[0178] In an embodiment of the invention, a dry particulate inert
filler may be incorporated into the build material that possesses
the ability to scatter or reflect the incident radiation without
absorbing it. It has been found that materials with this
characteristic enhance the absorption of radiation in the
absorber-treated regions without greatly affecting the
non-absorbing property of the untreated particulate build material.
By this means, less radiation is needed to facilitate melting,
sintering, or other forms of bonding in the absorber-treated
regions. This reduces the overall energy consumption, speeds the
process, and reduces the heat transmission to the untreated build
material adjacent to the treated regions in the build area.
[0179] Examples of diffractive and reflective inert materials will
be titania, zirconia and other metal oxide particles; high
refractive index glass, sapphire; aluminum, steel and other metal
dust; any particles that contain two or more materials with
significantly different refractive indexes-hollow glass beads,
core-shell glass beads
[0180] In an embodiment, an additive to the particulate build
material may comprise a plasticizer specifically adapted to the
thermoplastic or thermoset particulates. As used herein, the term
"plasticizer" denotes a chemical substance that that lowers the
melting point of the thermoplastic or thermoset particulate
material; or causes the thermoplastic or thermoset material to
possess a flow viscosity when melted that is lower than the pure
melted thermoplastic or thermoset material by itself. Plasticizers
may include, depending on the solubility parameter of the
thermoplastic or thermoset components of the build material,
mineral oils; phthalates, phosphates, adipates-dioctyl phthalate,
dioctyl adipate, diisononyl phthalate, dibenzyl phthalate,
dipropylene glycol dibenzoate, triaryl phosphate ester; epoxidized
soybean oil, glycerol, propylene glycol, urea, ethoxylated
glycerol, butanediol, pentanediol, hexanediol, erythritol, xylitol,
and sorbitol.
Coupling Agent for Inorganic Filler
[0181] In order to obtain better mechanical properties of the
finished product, inorganic filler material may be treated with a
coupling agent or a coupling agent may be added to the
thermoplastic or thermoset component of the build material.
Suitable coupling agents may include or consist essentially of, for
example, silica-based, including silanes such as
3-isocyanopropyltrietyloxisilane,
3-glycidoxypropyltrimethoxysilane,
aminoethylaminomethyl)phenethyltrimethoxysilane,
1,3-bis(iodomethy)tetramethyldisiloxane,
diethylphosphatoethyltiethoxysilane,
3-methacryloxypropyltrimehoxysilane; metallo-organic titanates,
metallo-organic zirconates, aluminates, and others. The
metallo-organic coupling agent may be, for example,
alcoxytrimethacryl titanate, isopropyl triisostearoyl titanate,
neopentyl(diallyl)oxytrineodecanyl titanate,
neopentyl(diallyl)oxytrineodecanyl zirconate, or alkylacetoacetate
aluminum diisopropylate.
Soluble Adhesives
[0182] The process of bonding the build material by means of
absorber does not preclude the use of other bonding mechanisms
disclosed elsewhere, for example, in U.S. Pat. No. 5,902,441 and
U.S. Patent Publication No. 2005/0003189. In these publications, a
dry particulate adhesive is chosen that is at least partially
soluble in the printed fluid. These materials dissolve shortly
after the fluid defining a layer is printed, greatly increasing the
viscosity of the fluid and promoting adhesion between grains of
fillers. The presence of these materials serves a useful purpose
outside of the additional strength they provide: by increasing the
viscosity of the printed fluid that prevent the migration of fluid
outside of the boundaries of the article under construction.
Further, they may tend to immobilize the absorber at specific sites
between grains of thermally activated build material (thermoplastic
or thermoset) thereby focusing the energy delivery to those
locations.
[0183] Examples of at least partially water-soluble adhesives
include, but are not limited to polyvinyl alcohol, sulfonated
polyester polymer, sulfonated polystyrene,
octylacrylamide/acrylate/butylaminoethyl methacrylate copolymer,
acrylates/octylacrylamide copolymer, polyacrylic acid, polyvinyl
pyrrolidone, styrenated polyacrylic acid, polyethylene oxide,
sodium polyacrylate, sodium polyacrylate copolymer with maleic
acid, polyvinyl pyrrolidone copolymer with vinyl acetate, butylated
polyvinylpyrrolidone, polyvinyl alcohol-co-vinyl acetate, and
combinations and copolymers thereof.
Active Fillers
[0184] Articles formed by selective absorption sintering may have
edges that curl up out of a plane in which they are printed because
of differential cooling rates between build materials and
absorbers, or because of capillary contraction of melted or
sintered aggregates in the build material. This may cause a loss of
accuracy in the part being formed, and may cause the printing
process to fail because the curled layers may be caught by the
leveling mechanism during the deposition of a layer of build
material.
[0185] Polymeric build material may be mixed with one or more
resins and/or cements, generally termed "active" fillers. In a
preferred embodiment, the resins or cements are water soluble. The
resin or cement content may be from 1% to 99% by weight of the
build material. The purpose of the active filler is to provide the
printed layer with short term strength sufficient to resist the
forces created by differential cooling after sintering the layer.
Some or most of the final part strength may be derived from the
material properties of the polymer and not from the resin or cement
comprising the active filler.
[0186] As used herein, the term "active filler" comprises a
component of the particulate build material that participates in a
chemical reaction that is initiated by the presence of the carrier
fluid or binder deposited within a layer. In U.S. Patent
Publication No. 2004/0056378 A1, a method for forming
three-dimensional objects is disclosed that incorporates a
two-stage hardening mechanism to eliminate distortion during
curing. Embodiments of the present invention are compatible with
the methods disclosed in the application referenced above. For
example, in accordance with the process described in U.S. Patent
Publication No. 2004/0056378, a separate chemical reaction between
filler components or a filler component and the printed fluid may
create a solidified network in a short period of time before the
heating step is performed in accordance with an embodiment of the
present invention. These components are called "active" fillers.
The solidification or bonding disclosed herein may constitute the
second-stage hardening mechanism of a two-stage process as
disclosed in the above-referenced application. The active filler is
chosen such that it forms a solid network or gel within the printed
regions of the build material that resists further stresses imposed
by, for example, capillary attraction, evaporation shrinkage, or
distortions resulting from melting, sintering, or crosslinking of
the thermoplastic or thermoset components used in aspects of the
present invention.
[0187] The active filler may include or consist of an inorganic
adhesive, such as at least one of plaster (accelerated by any of a
number of accelerators including terra alba, sodium chloride,
potassium chloride, ammonium chloride, under-calcined plaster,
alum, potassium sulfate, potassium aluminum sulfate, ammonium
sulfate, sodium sulfate, calcium hydroxide, calcined lime, sodium
tetraborate, potassium nitrate, ammonium oxalate, ammonium nitrate,
magnesium sulfate, barium sulfate, or aluminum sulfate), bentonite,
sodium silicate, salt, Portland cement, magnesium phosphate cement,
magnesium chloride cement, magnesium sulfate cement, zinc phosphate
cement, calcium phosphate cement, zinc oxide-eugenol cement, and
combinations thereof.
[0188] A preferred embodiment includes no more active filler than
necessary to counteract the forces created during cooling of the
sintered layer, such that the properties of the resin or cement
comprising the active filler do not dominate the properties of the
finished part. The preferred level is below 50% by volume, and most
preferably 30% by volume or less. Volume percents here are
determined by bulk density. Since inorganic fillers generally
possess a much higher density than organic materials, the volume
percentage is most meaningful here.
[0189] In a preferred embodiment, the liquid carrier for the
absorber that is printed acts as a solvent or catalyst for the
active filler in the solid build material so that the absorber for
the sintering energy can be delivered at the same time as the
solvent for the resin or cement comprising the active filler. This
is typically an efficient method for creating a part.
[0190] In another preferred embodiment, the geometry printed with
solvent or catalyst for the active filler is the same as the
geometry that is sintered.
Thermoplastic and Thermoset Coatings on Fillers
[0191] When 100% thermoplastic particulate materials are subjected
to temperatures above the glass transition and melting point
temperatures, the thermoplastic particle may distort from volume
expansion and then shrinkage as it liquefies, or it may migrate
through the pores of the particulate build material by capillary
attraction. These motions, if they happen in a uncontrolled way,
can lead to distortion of layers of sintered material. This type of
distortion may decrease the accuracy of articles created from
three-dimensional printing material systems utilizing thermoplastic
and thermoset particulate additives. Articles may be heated to high
temperatures at or above a melting point of the thermoplastic to
acquire the toughness and strength of the thermoplastic or
thermoset additive as it melts and fuses together.
[0192] Coating inert particles with a liquid coating that is
thermoplastic or thermoset when dried may be a way to decrease the
amount of distortion the coating may undergo when subjected to high
temperatures above the glass transition temperature and melting
point of the coating. The inner inert phase of the particle may
exhibit significantly less distortion when subjected to the heat
required to melt the coating phase and provide extra stability to
the thermoplastic or thermoset phase with respect to fluid flow
from melting. The distortion on melting is confined to the thin
thermoplastic coating on the surface of the particles. The apparent
viscosity is a bulk property that is the combination of rigidity of
the underlying particle and the flow of the coating, and this
property generally increases for systems containing large fractions
of inert fillers.
[0193] The filler substrate for the thermoplastic or thermoset
coating may be regarded as a specific form of the abovementioned
fillers or pigments. The build material may comprise grains of a
first material that are coated with a layer of a second material,
wherein the thickness of the layer is such that the resulting
particulate material, containing this combination of the first
material and second material coating, has a grain size as discussed
above. The second material, which makes up the coating of the
grains of the first material, is preferably less susceptible than
the selected absorbers to direct heating by electromagnetic
radiation or electromagnetic induction as described above. The
second material preferably also exhibits sufficient flowability in
the heated state, and is preferably capable of melting or sintering
on exposure to heat, the heat being that provided by the
absorber.
[0194] Typically, when describing coated particles, their
components fall into one of two categories: core and coating, with
simply coated particles having one of each. In some embodiments,
multiple coatings may be applied in successive shells.
Agglomerates, i.e., grains with multiple core particles, are
occasionally also grouped into this category. Coated materials need
not possess a uniform grain size or coating thickness. A coating
process may possibly yield a build material that is structured as
agglomerates of the first and second materials intermixed; the
coating process functioning optionally as a particle-enlargement
process as well as a coating process.
[0195] Coating materials that may be used include, but are not
limited to, the above-mentioned polymers or copolymers, preferably
selected from polyester, polyvinyl chloride, polyacetal,
polypropylene, polyethylene, polystyrene, polycarbonate,
polybutylene terephthalate, polyethylene terephthalate,
polysulfone, polyarylene ether, polyurethane, polylactides,
thermoplastic elastomers, polyoxyalkylenes,
poly(N-methylmethacrylimide) (PMMI), polymethyl methacrylate
(PMMA), ionomer, polyamides, copolyester, copolyamides, silicone
polymers, terpolymers, acrylonitrile-butadiene-styrene copolymers
(ABS), polyether sulfone, polyaryl sulfone, polypheylene sulfide,
polyaryl ether ketone, polyphthalamide, polyimide,
polytetrafluoroethylne, mixtures thereof, or phenolic resins.
[0196] The particulate build material may include inorganic
particles coated with a liquid coating that is thermoplastic or
thermoset when dried. The coating may be deposited from either a
solvent-based solution or an aqueous dispersion/emulsion. In an
embodiment, the dried coatings preferably have a sufficiently low
softening/melting point to be effectively heated sintered or fused
at a reasonable temperature. The temperature ranges are nearly the
same as for thermoplastic material, from about 50.degree. C. to
350.degree. C.; preferably from about 70.degree. C. to 200.degree.
C. Such polymers, e.g., polyurethanes, are available as aqueous
dispersions and are good candidates because of the range of
softening/melting points that can be acquired.
[0197] Inorganic core filler materials have relatively high surface
energy and provide good adhesion of the thermoplastic outer coating
onto the surface of the inorganic particle. Rough, irregular, and
porous fillers may provide better adhesion to the thermoplastic
than round, spherical, non-porous particles. Surface treated
particles, using coupling agents as described above, e.g.,
amino-silane coated soda-lime glass, may also increase adhesion of
the outer coating to the core.
[0198] Typical inorganic fillers suitable for forming the core of
the coated particle include metallic oxides such as titanium
dioxide, magnesium oxide, zinc oxide, aluminum oxide, silica
(amorphous, fumed, or crystalline), soda-lime glass, borosilicate
glass, calcium carbonate, magnesium carbonate, gypsum, talc, clay,
boron nitride, olivine, calcium silicate, magnesium silicate,
ceraminc, aluminosilicates, amino-silane surface-treated soda lime
glass, epoxy-silane treated soda-lime glass, amino-silane treated
borosilicate glass, epoxy-silane treated borosilicate glass, and
amino-silane surface treated calcium silicate, limestone, plaster,
bentonite, precipitated sodium silicate, amorphous precipitated
sodium silicate, amorphous precipitated lithium silicate, salt,
aluminum hydroxide, magnesium hydroxide, calcium phosphate, sand,
wollastonite, dolomite, amorphous precipitated silicates containing
at least two types of ions selected from sodium ions, lithium ions,
magnesium ions, and calcium ions. Any of the fillers listed above
as "inert" fillers may be used as substrates for coatings of
thermoplastic or thermoset materials. Appropriate combinations of
materials may be evident to those experienced in the art.
[0199] A typical coating that may be applied as a liquid and dried
to form a thermoplastic includes or consists essentially of, for
example, any of the following: aqueous aliphatic urethane
dispersions, aqueous acrylic emulsions, aqueous dispersion of
ethylene acrylic acid copolymers, solution aliphatic urethanes,
solution polymethylmethacrylates, solution polyethylmethacrylate,
solution polybutylmethacrylate, ethylene vinyl acetate emulsions,
alcohol soluble polyamides, sulfonated polyester, and
cyanoacrylates.
Particle Coating Techniques
[0200] Coating of particulate materials provides a means of
combining multiple chemistries in each individual grain, and may be
accomplished by a variety of methods. A few examples of these
methods are fluid bed coating, hybridization, and spinning disk
coating. Fluid-bed coating is most commonly preferred when a
uniform coating is desired on substrate particles that are uniform
in size. Hybridization is a process by which small grains of
coating materials are agglomerated on the exterior of larger
substrate grains. The disk coating process can yield uniform
coatings on substrate grains, or under different conditions, it can
yield agglomerates of uniformly mixed grains of substrate and
coating material. The choice of coating method depends on the
desired application and the properties and size distribution of the
raw materials.
[0201] A method for coating particles is fluid bed processing, in
which a charge of dry substrate particles is fluidized in a hot
moving air stream, and a solution of coating material is sprayed
into the charge, typically by means of a pressure or two fluid
nozzle. The atomized droplets of coating material adhere to the
surfaces of the substrate particles, and the solvent carrier is
evaporated in the hot air stream leaving a dry film of coating. In
an efficient process, the air stream is heated to a level above the
boiling point of the solvent, but low enough so as to not dry the
droplets of coating material before they adhere to the core
particles, as well as not degrade either of the component
materials. This method may be adapted to make use of molten coating
materials, rather than those dissolved in solvent by chilling the
fluidizing air stream, provided the viscosity of the melt is low
enough to spray. This process is compatible with both thermoplastic
and thermoset materials, provided these materials can be dissolved
in a solvent that is compatible with the processing method, or if
the melt viscosity is low enough to facilitate the molten coating
process at temperatures that are compatible with the processing
equipment.
[0202] In the hybridization coating process, small grains of
coating material (typically around 1-5 .mu.m in size) are mixed
with larger substrate grains (typically 50-300 .mu.m in size) and
the mixture is subjected to an energetic shearing action. Particles
of the finer coating material adhere to the surface of the
substrate grains by electrostatic attraction, or by a quiescent
chemical bonding if such exists between the coating and the
substrate. This type of coating method is particularly useful for
dry-blending of particulate materials that are capable of
chemically reacting once they are moistened by the printed fluid,
but react only very slowly in the dry state.
[0203] In spinning-disk coating, the substrate and filler grains
are dispersed together in a liquid carrier. Typically, the liquid
carrier is a solvent for the coating material, and a non-solvent
for the substrate. The resulting slurry is sprayed onto the surface
of a rapidly rotating disk that atomizes the slurry and sprays it
into a chamber with rapidly agitated hot air. The solvent is
evaporated from the droplets of slurry, resulting in solid
particles of substrate covered with the coating material that falls
out of solution when the liquid carrier evaporates. Depending on
the processing conditions, the droplets of atomized slurry may
contain one or several particles of substrate. This process is
compatible with both thermoplastic and thermoset materials,
provided these materials can be dissolved in a solvent that is
compatible with the processing method.
[0204] Thermoset coatings can be co-extruded with inert filler
particles at a temperature below the crosslink temperature of the
thermoset material. Thermoplastic coatings may be formed in the
same way, with the processing temperature above the melt flow
temperature of the thermoplastic. The extruded material can undergo
coarse and fine milling operations followed by particle size
classification in order to achieve the desired grain size
distribution. The structure of the grains may be a disordered
mixture of two phases rather than discrete particles with uniform
coatings.
Absorbing Filler Applications
[0205] In yet another embodiment, the absorber may be the
particulate build material itself, e.g., calcium sulfate. As
mentioned in U.S. Pat. Nos. 5,328,539 and 5,182,134, CaSO.sub.4 may
be used as an absorber for electromagnetic induction. An aqueous
ink may be applied onto a layer of a plaster-based build material
in areas where the two-dimensional cross-section of the
three-dimensional article is to be formed. Each successive layer
may be exposed to induction that heats up the calcium sulfate to
drive off the water from the printed regions. This procedure dries
each layer so that the article may be removed relatively
quickly.
[0206] In another embodiment, an article is created from a material
system where either the absorber is applied as an ink via an inkjet
printhead, or is included and evenly distributed in the build
material, like calcium sulfate in Z Corp products zp.RTM.100 and
zp.RTM.130. The article may then be infiltrated with a
heat-activated infiltrant resin such as epoxy/amine resins like
ZMaX.TM. or ZSnap.TM. from Z Corporation. The article may then be
placed in an induction oven, and exposed to low frequency induction
to evenly heat and cure the infiltrated article
Flow Properties of Build Materials
[0207] Compositions have been disclosed that relate to control of
the flow properties of the build material in Three Dimensional
Printers. The three principal methods are the addition of liquid
"processing aids," control of grain size distribution, and the
addition of solid fillers that contribute to the frictional
behavior of the build material. Many candidate materials have been
disclosed previously, for example, in U.S. Patent Publication No.
2005/0003189. Previously, however, the exact implementation of
these methods has been by trial and error. Here, some mechanical
properties of dry particulate build materials are disclosed that
are particularly suited for use in Three Dimensional Printing,
especially in contrast to other formulations of similar materials
for other uses that may not require special flow characteristics of
the raw materials.
[0208] Referring to FIG. 4, in an embodiment of a three dimensional
printer, dry, free-flowing particulate build material is spread by
a rotating spreader rod, i.e., spreading roller 2. The rod rotates
in a direction co counter to a direction of motion of the spreading
mechanism. A circulating bead 50 of build material 32 is pushed in
front of a moving rod over a stationary bed. For the sake of
convenience, the system is shown in the frame of the rod with a
moving bed 51 and stationary bead. The bed is assumed to approach
the spreader in a direction u, and the bead of build material
circulates around a nearly stationary center. One may assume that
the build material is lifted by the leading surface of the spreader
rod because it adheres to a rod surface 52. The direction of the
flow of the build material reverses close to a nip 54, i.e., an
interface between the spreading roller 2 and the moving bed 51.
[0209] The equilibrium of a small printed feature as it passes
directly underneath the spreader rod is analyzed. On typical 3D
Printers, a thickness t of a single printed layer of build material
32 is approximately 1/100 the radius a of the spreader rod.
Referring to FIG. 5, the spreader exerts a compressive stress
.sigma..sub.zz and a shear stress .tau..sub.xz on the build
material directly underneath it. There is also a horizontal stress
component .sigma..sub.xx.
[0210] One may assume that the horizontal stress applied to a left
edge of a feature 56 of an article is not opposed by another stress
on a right edge. The feature is assumed to leave a wake 58 behind
it where build material, after being swept along the upper surface,
is unable to wrap around the downstream corner and establish a
stress analogous to hydrostatic pressure against the right surface.
The horizontal stress applied to the left may be opposed by a shear
stress along s bottom surface. A free body diagram of the feature
is shown in FIG. 5b, including a hollow cavity 60 formed in the
feature wake 58.
[0211] It is assumed here that dry, free-flowing particulate build
material in motion possesses a different shear strength than build
material that has been allowed to rest for a time. In general, one
may expect a different yield locus for build material in different
states of motion. For purposes of this derivation, this is
expressed here as two different sets of yield parameters, "static"
and "dynamic" values of the cohesion and friction angle.
[0212] These properties of granular materials are amply supported
in the literature. See, for example, B. M. Das, Advanced Soil
Mechanics, Hemisphere Pr. 1997, pp. 315-317 or S. Aranson & L.
S. Tsimring in The Physics of Granular Media, H. Hinrichsen &
D. Wolf, eds, Wiley-VCH, (2004) pp. 146-147, incorporated herein by
reference in their entireties.
[0213] A force balance on the feature shown in FIG. 6 leads to the
equation: 1[c.sub.s-c.sub.d+.sigma..sub.zz(tan .phi..sub.s-tan
.phi..sub.d)]=L.DELTA..tau.>t.sigma..sub.xx (1) for the feature
to remain in place. The normal stress against the bottom surface of
the feature is assumed the same as that against the top surface.
The difference in shear strength between the static values (static
yield locus 60) and dynamic values (dynamic yield locus 62) with
normal stress .sigma..sub.zz is denoted by .DELTA..tau..
[0214] "Bredt flow parameter" (Br) is herein defined, expressing,
in general, the propensity for printed features to shift in the
build area of a three dimensional printer during spreading of build
material: .DELTA..tau./.sigma..sub.xx=Br>t/L.apprxeq.0.1 (2)
[0215] The ratio t/L is slightly arbitrary. One may assume for
practical purposes that features with a length at least several
times the layer thickness (L.about.10 times t) are those that are
preferably considered in this model. Layers with thickness of 100
micrometers are standard in three dimensional printing machines
that are currently available, and instability of isolated patches
smaller than 1.0 mm may have a minimally discernable effect on the
appearance of a model.
[0216] For the flow conditions most useful for three dimensional
printing, the build material is non-cohesive, i.e., the cohesion of
the granular material is much less than the dynamic pressure of
material in flow. Using reasonable values for the bulk density of
the build material and spreading speed in a standard
ZPrinter.RTM.310 three dimensional printer, one obtains an order of
magnitude estimate:
c.sub.s.apprxeq.c.sub.d<<.rho.(u+.omega.a).sup.2.apprxeq.600
Pa (3)
[0217] A material having shear strength of this magnitude is a weak
gel such as yogurt. While it is not "strong" in any sense of the
word, it is by no means "free-flowing." As an additional estimate
of the lower bound of the cohesion, we may observe that the bead of
free-flowing particulate build material may be in a state of
yielding at the bottom of the pile when the counter-roller begins
to move it across the build area. In a ZPrinter.RTM.310 three
dimensional printer, the bead is approximately 1 cm tall.
Accordingly, the following inequality holds:
c.sub.s.apprxeq.c.sub.d<<.rho.gh.apprxeq.100 Pa (4)
[0218] This is typically a minimum acceptable range for cohesion in
a particulate build material for it to be considered
"free-flowing." While the compressive and shear stress imposed on
the build material through the motion of the counter-roller may
have a magnitude approximately 600 Pa, the cohesion is preferably
accordingly less than 100 Pa in order for it not to adversely
affect the layering of build material.
[0219] With the assumption that the cohesion is negligibly small,
the following simplification can be made. (tan .phi..sub.s-tan
.phi..sub.d)>t.sigma..sub.xx/L.sigma..sub.zz (5) and .sigma. xx
.sigma. zz = ( 1 + sin .times. .times. .PHI. d ) ( 1 - sin .times.
.times. .PHI. d ) ( 6 ) ##EQU1##
[0220] This leads to an equation ( tan .times. .times. .PHI. s -
tan .times. .times. .PHI. d ) .times. ( 1 - sin .times. .times.
.PHI. d ) ( 1 + sin .times. .times. .PHI. d ) = Br nc > 0.1 ( 7
) ##EQU2##
[0221] Equation 6 expresses a vitally important feature of
free-flowing particulate build materials that are suitable for use
in three dimensional printing machines. The quantity on the left is
termed the "Bredt flow parameter for noncohesive particulate
materials," and it preferably has a value greater than about 1/10
for small printed features to remain stationary during
spreading.
Measurement of Static and Dynamic Friction Coefficients
[0222] Methods exist for measuring the static yield properties of
granular materials in shear. See, for example, B. M. Das, Advanced
Soil Mechanics, Hemisphere Pr. 1997, pp 313-326. It is found,
however, that the values for the yield parameters .phi. and c vary
with experimental conditions, and it is necessary to measure the
properties in stress range of interest.
[0223] An example of a piece of laboratory equipment that is
capable of measuring the static friction characteristics of
particulate materials is the "ShearScan TS12" manufactured by
Sci-Tec Inc. This device holds a sample of material in a
cylindrical cell and applies a vertical load to the material to
consolidate it to a specified level. The device then applies a
gradually increasing transverse shearing force until it detects
slip in the sample of material. It performs this measurement across
a range of applied loads to develop a yield locus analogous to
those pictured in FIG. 3. Since the instrument measures the shear
stress at the instant of rupture, this is the "static" friction in
the particulate material.
[0224] One difficulty in this analysis with the ShearScan
instrument is that it is designed to measure the frictional
characteristics of particulate materials in large silos when they
are subjected to stress levels much larger than that found in the
spreading system of a 3D Printer. The stress was estimated in
equation (3) above to be on the order of 1/2 kPa, about 1/10 the
stress levels in the operating range of the ShearScan.
[0225] Furthermore, there does not exist an analogous instrument to
measure the "dynamic" friction characteristics of particulate
materials. Several instruments called "powder rheometers" exist,
for example the FT4 Powder Rheometer manufactured by Freeman
Technology. This device doesn't strictly measure a yield locus,
however. It measures the rate of working of a particulate material
in a specialized mixing cell where the stresses in the sample are
not known. It is therefore not suitable for use in this model.
[0226] An approximate laboratory procedure may provide estimates of
the flow parameter for non-cohesive particulate build materials.
This is done by measuring the angle of repose of a pile of a
particulate material under static and dynamic conditions. The
procedure is accomplished as follows. On a metal sheet, a conical
pile is formed from a particulate material sample by sprinkling
particles very slowly over one point from a height of about 1 cm
above the growing top of the pile. The diameter d and height h of
the pile are measured. The ratio d/2h is an approximate measure of
the static friction coefficient tan .phi..sub.s. Next, a small
piece of metal, such as a screwdriver, is used to tap lightly on
the plate so the pile collapses. The height and diameter are
measured again, and the ratio d/2h is an approximate measure of the
dynamic friction coefficient tan .phi..sub.d.
[0227] The height of the pile is chosen such that
gh.apprxeq.(u+.omega.a).sup.2
[0228] This ensures that the stress at the bottom of the heap is in
approximately the right range. For ordinary 3D Printers
manufactured by ZCorp, this height is roughly 5 cm. It is necessary
to tap the plate relatively lightly so that the motion of the pile
after the tapping is primarily driven by gravity, and not by
kinetic energy from the tapping motion. One or two light taps may
be sufficient.
[0229] Several particulate samples were measured in this manner,
and the data are presented below. The calculated flow parameter is
the "noncohesive" form given in equation 6. TABLE-US-00001 TABLE 1
Measurements of flow parameter for various candidate particulate
build materials particulate sample tan phi s tan phi d Br(nc) zp100
0.829268 0.52381 0.111833 zp100 0.909091 0.45 0.191933 zp100 1
0.654545 0.100967 zp130 0.65 0.348837 0.151927 zp130 0.742424
0.397059 0.159174 zp130 0.789474 0.454545 0.138869 4F lucite
0.53125 0.28169 0.143096 50 .mu.m Al.sub.2O.sub.3 0.639344 0.439394
0.08523 Coated glass beads 0.45 0.349206 0.050812 +10 ppm Neobee
M20 0.461538 0.323529 0.073043 +20 ppm Neobee M20 0.516129 0.328571
0.09832 +30 ppm Neobee M20 0.671642 0.527778 0.052302 +40 ppm
Neobee M20 0.786885 0.693548 0.025571 +50 ppm Neobee M20 0.781818
0.763636 0.004448 zp100 and zp130 are products marketed by ZCorp
for building appearance models. 4F Lucite from Ineos Acrylics has a
particle size between 55 .mu.m and 70 .mu.m. Tabular 50 .mu.m
Al.sub.2O.sub.3 acquired from KC Industries Glass Beads from
Potter's Industries, 72 .mu.m grain size, aminosilane surface
treatment Neobee M20 was used to coat glass beads. Neobee M20 from
Stepan Industries
[0230] As these data approximately show, build materials designed
by Z Corp for three dimensional printing all fall in the same
range, a little bit higher than the required lower bound. Some
scatter in the results is to be expected with this approximate
technique. Although the static angle of repose of zp100 is higher
than in zp130, the flow parameter for the two build materials is
nearly the same. In fact, qualitative experience shows that these
two products perform about the same.
[0231] Of the other three materials tested, glass spheres had the
poorest performance, with a flow parameter of only 0.05. This, too,
is supported by qualitative experience, glass beads alone are
unsuitable for 3D Printing from the standpoint of spreading.
[0232] To illustrate the extreme sensitivity of particulate
behavior with even small additions of certain chemicals, generally
referred to as "processing aids," a series of data were taken in
which tiny (10 ppm) increments of a low-viscosity emulsifier are
added to a sample of glass spheres. The flow parameter rises
quickly, peaks, and falls away even more quickly even though both
the static and dynamic friction angles increase through the series.
The critical point occurs when the dynamic angle of repose
transitions from a nearly constant value to a linearly increasing
value. This shows that there can be rather sharp optima in
composition to obtain useful spreading characteristics.
[0233] This test is a fairly useful technique for identifying
relative performance properties between different candidate
materials. The preferred method for evaluating flow properties of
candidate build materials during formal optimization after the
initial selection period is to test samples of the material on a
working three dimensional printer. Certain pathological geometries
are known to those experienced in the art, and they can be
evaluated either qualitatively or quantitatively. One particularly
useful part for observing stability during spreading is a flat
plate studded with pegs that are oriented downward during the
build. During printing, the earliest layers addressed are a series
of disconnected patches that are relatively free to shift in the
build material. After these have been formed, a plate is printed
that joins all of the pegs together in a single object. One can
easily examine whether the pegs are uniform and straight, and one
can evaluate the quality of spreading on that basis.
Liquid Flow Aids
[0234] The build material may include a processing aid, such as a
viscous liquid and/or a polymer having a low melting point. The
processing aid material may include or consist essentially of at
least one of the following materials: polyethylene glycol,
polypropylene glycol, sorbitan monolaurate, sorbitan monooleate,
sorbitan trioleate, sorbitan sesquioleate, polysorbates, poly
(ethylene oxide) modified silicone, poly (propylene oxide) modified
silicone, secondary ethoxylated alcohols, ethoxylated nonylphenols,
ethoxylated octylphenols, C.sub.8-C.sub.10 alcohols,
C.sub.8-C.sub.10 acids, polyethylene oxide modified acetylenic
diols, citronellol, ethoxylated silicones, ethylene glycol
octanoate, ethylene glycol decanoate, ethoxylated derivatives of
2,4,7,9-tetramethyl-5-decyne-4,7-diol, polyoxyethylene sorbitan
mono-oleate, polyethylene glycol, soybean oil, mineral oil,
fluoroalkyl polyoxyethylene polymers, glycerol triacetate, oleyl
alcohol, methyl oleate, isopropyl palmitate, other hydrophobic
fatty-acid esters, oleic acid, squalene, squalane, essential oils,
esters, terpenes, greases, or waxes, propylene glycol, ethylene
glycol, C.sub.8-C.sub.10 esters of mono-, di-, or triglycerides,
fatty acids, ethoxylated fatty acids, lecithin, modified lecithins,
unsaturated mono- and diglycerides, distilled acetylated
monoglycerides, diacetyl tartaric acid esters of mono- and
diglycerides, polyglycerol esters, polyglycerol polyricinoleate,
and combinations thereof.
[0235] It is worth noting that many of these flow aids may also
function as plasticizers for the thermoplastic or thermoset
components of the build material.
[0236] These liquid additives may be mixed with the dry build
material at relatively low weight fractions, most preferably less
than 0.5 percent by weight relative to the solid fraction.
Generally, a very small addition of liquid is all that is permitted
in order to prevent the build material from becoming cohesive, and
therefore, not free-flowing. This cohesive yield strength was
estimated in equation (4) above to be around 100 Pa.
[0237] Some processing aids are extremely effective on the friction
low levels below the concentration where it affects the cohesion to
any significant degree. This is demonstrated by the use of Neobee
M20 on glass beads as shown in Table 1. While the precise nature of
the phenomenon leading to changes of the Bredt flow parameter with
varying content of processing aid is not known, it is hypothesized
that the cohesive force between stationary particles is more
greatly affected by the presence of a liquid processing aid than
the viscous forces between particles in motion.
[0238] Besides their effect on the frictional flow properties of
the granular build material, processing aids are chosen on the
basis of chemical compatibility with the chemical nature of the
build material, their ability to reduce dust emissions from the
operating machine, and their ability to influence to flow of the
printed fluid (liquid binder or absorber in liquid carrier) through
the pores in the granular build material. In particular, it is
desirable that the printed liquid remain closely associated with
the location where it was printed, and it is desirable that the
processing aid be slightly repellant to the printed liquid to help
arrest its migration through the pores in the loose build material
adjacent to the printed regions.
[0239] In the preceding sections, the term "dry free-flowing
particulate build material" has been used extensively to describe
the inventive substance used for constructing articles. The
presence of small quantities of nonaqueous liquids does not affect
this definition. The upper limit to the quantity of a flow aid is
determined by the degree of cohesion it imparts to the build
material. Additions that are excessive prevent the build material
from flowing freely, but when the correct amount is used, the build
material flows freely, and it may be considered "dry" in a
mechanical sense for purposes of embodiments of this invention. It
is the operating range of the "Bredt" flow parameter that
determines the limits to what may be considered "dry" build
material.
Grain Size Distributions
[0240] In one embodiment of the invention, the granular components
of the build material have a median grain size (d.sub.50) of from
about 5 to about 150 micrometers, preferably from about 20 to about
100 micrometers and more preferably from about 40 to about 70
micrometers. Depending on the application, however, build materials
comprising smaller particles, and also those comprising larger
particles, may be used. Three-dimensional articles with preferred
resolution and surface smoothness may be obtained using particles
whose median particle size is from about 10 to about 45
micrometers, preferably from about 10 to about 35 micrometers, and
more preferably from about 20 to about 30 micrometers.
[0241] Difficulties may be encountered in the processing of fine
materials having a d.sub.50 smaller than 20 micrometers, and in
particular smaller than 10 micrometers, because these particles do
not flow well, and bulk densities significantly decrease. These
features can increase the porosity in the final object. For optimal
spreading characteristics, it may be advantageous to use particles
whose median size d.sub.50 is from about 60 to about 150
micrometers, preferably from about 70 to about 120 micrometers, and
more preferably from about 75 to about 100 micrometers.
[0242] The aforementioned properties of build materials for three
dimensional printing are not entirely compatible with one another,
and so for materials that are commercially useful in three
dimensional printing machines, the grain size distribution most
often represents a compromise between better accuracy and more
reliable layering of the build material. Most typically, it is a
relatively broad grain size distribution that covers the full range
between 10 micrometers and 150 micrometers with a median grain size
preferably between 30 micrometers and 75 micrometers that has
optimal performance. As stated above, the presence of a processing
aid can have a dramatic effect on the flow properties of the build
material, and the optimal grain size distribution is highly
dependent on the choice of processing aid.
[0243] During formulation of a particulate build material, the
density of compacted layers is preferably taken into account
because of the way in which it affects strength and distortion of
the final part. Addition of fine grains in a controlled amount may
have a beneficial effect on the flow parameter, and so a balance is
preferably struck such that the flow parameter is maintained with
an acceptable range while maintaining a high density in the
material produced.
[0244] In one embodiment of the invention, the dry particulate
build material preferably comprises a particulate material prepared
by milling, precipitation, and/or anionic polymerization, or by a
combination of these processes. In a preferred embodiment, the
build material comprises a precipitation of somewhat excessively
coarse particles that have been subsequently milled, or a
precipitation of particles that have been subsequently classified
to adjust the particle size distribution.
Fine Fillers as Flow Aids
[0245] One or more particulate additives may be used to improve
handling and spreading of the build material. By way of example,
these additives may act as flow aids. The may comprise from about
0.05% to about 5% by weight, and preferably from about 0.1% to
about 1% by weight, of additives, based the total weight of the of
the components of the Flow aids include, but are not limited to,
fumed silicas, stearates, or other flow aids known from the
literature, for example, tricalcium phosphate, calcium silicates,
Al.sub.2O.sub.3, MgO, MgCO.sub.3, or ZnO. By way of example, fumed
silica is supplied by Degussa AG with the trademark
Aerosil.RTM..
[0246] Any of the ingredients listed above as potential inert
fillers or substrates for thermoplastic coatings may be used in low
percentages as flow aids if they possess frictional characteristics
that affect the flow parameter in a beneficial way. Many of the
most effective particulate flow aids are fibers of natural
polymers, modified natural polymers, synthetic polymers, or
ceramics. Several particularly beneficial inert fillers are
inorganic materials that may include or consist essentially of
plaster, terra alba, bentonite, calcium silicate, calcium
phosphate, magnesium silicate, magnesium phosphate, aluminum oxide,
aluminum hydroxide, limestone, dolomite, wollasonite, mica, glass
fiber, glass powder, cellulose fiber, silicon carbide fiber,
graphite fiber, aluminosilicate fiber, and mineral fiber. Some
preferred inert fillers are organic fillers that may be useful as
flow aids include starch, modified starch, maltodextrin, cellulose,
polypropylene fiber, polyamide flock, rayon, polyvinyl alcohol
fiber, sugars and sugar alcohols, especially sucrose, lactose,
mannitol, sorbitol, xylitol, and maltitol; and carbohydrates such
as acacia gum, locust bean gum, pregelatinized starch,
acid-modified starch, hydrolyzed starch, sodium carboxymethyl
cellulose, sodium alginate, hydroxypropyl cellulose, methyl
cellulose, chitosan, carrageenan, pectin, agar, gellan gum, gum
Arabic, xanthan gum, propylene glycol alginate, guar gum, and
combinations thereof.
Antistatic Additive
[0247] The polymeric particulates described herein typically have
low dielectric constants and, therefore, may easily build up static
charge. This can cause uneven spreading of the build material and
agglomeration of the particles. Use of antistatic additives may be
highly beneficial. Examples of antistatic agents are glycerol
stearate, alkyl sulfonate, and ethohylated amine sold under the
name ATMER 261 by Ciba.
[0248] Having generally described features of embodiments of the
invention, a further understanding may be obtained by reference to
certain specific examples, which are provided herein for purposes
of illustration only, and are not intended to be limiting unless
otherwise specified.
EXAMPLES
Example 1
[0249] Production of a tensile specimen using polyolefin based
composition.
[0250] Using the apparatus described above, a tensile specimen was
made using a dry free-flowing particulate composition containing
17.0% of polypropylene (Microthene FP-8090 average particle size 20
.mu.m) and 83.0% of aminosilane-modified glass beads (Potters
Industries 3000E) as a build material. The absorber was zb.RTM.56
binder from Z Corporation containing 2.0% solids by weight of
chemically modified carbon black (Cab-O-Jet IJX352B). A total of 16
layers were printed, and each layer in turn was irradiated by a
500-watt tungsten-halogen lamp (5'' length) that traveled over the
build area at a speed of 17.7 mm/sec. Layer thickness was 0.10 mm
and volume fraction of the absorber fluid was 0.19.
[0251] Final part had tensile strength of 5.76 MPa and elongation
at break of 4.5%
Example 2
[0252] Production of a tensile specimen, a flexural strength
specimen and a 50-layer thick part using thermoset epoxy containing
inert filler.
[0253] In this example, the build material consisted of 29.6%
granulated epoxy: Everclear.RTM. EFC500S9 from Dupont, and 70.4% by
weight of 75 .mu.m diameter glass beads (Potter Industries 3000E
grade). Absorber fluid, volume fraction of the absorber fluid and
build layer thickness and were the same as in Example 1. The
tungsten-halogen lamp used in Example 1 was traversed over each
printed layer at a speed of 20.3 mm/sec. At the completion of the
build process, the partially bonded object was removed from the
surrounding unbound material and heat-treated in the convection
oven for 2 hours at 95 Celsius. After heat treatment the flexural
strength of the material was 54 MPa; tensile strength 35 MPa,
elongation at break 2.2%. A 50-layer part had good dimensional
stability but had issues with caking (build material from the
unprinted areas melted on the surface of the part)
Example 3
[0254] Production of the tensile specimen using a fast-drying PVA
system.
[0255] In this example the build material consisted of 20% milled
polyvinyl alcohol with a mean grain size approximately 100 microns,
and 80% of aminosilane-modified glass beads glass beads (Potter
Industries "Spheriglass" 2530 CP03 grade). Absorber fluid, volume
fraction of the absorber fluid and build layer thickness and were
the same as in Example 1. The tungsten halogen lamp used in Example
1 passed over each printed layer twice at the speed of 12.7 mm/sec.
Tensile strength of the specimen was 6.4 MPa and elongation at
break 2.6%. The same material was printed without passing the light
source over the printed layer. After 16 hours drying in the
printing bed the part was soft and impossible to handle.
Example 4
[0256] Production of the tensile specimen from a
thermoplastic-thermoset composite system.
[0257] In this example the build material consisted from 13.8%
milled Topas 5010L (average particle size 85 .mu.m) and 10.0%
granular epoxy: Everclear.RTM. from Dupont and 76.2% of glass beads
(Potter Industries 3000E grade). Absorber fluid, volume fraction of
the absorber fluid and build layer thickness were the same as in
Example 1. The tungsten halogen lamp used in the Example 1 passed
over each printed layer twice at a speed of 6.4 mm/sec. At the
completion of the build process, the partially bonded object was
removed from the surrounding unbound material and heat-treated in a
convection oven for 16 hours at 100 Celsius. After heat treatment
the tensile strength 6.9 MPa, the elongation at break was 3.6%.
Example 5
[0258] Using a laser diode for the selective sintering.
[0259] In this example both the build material and the absorber
were the same as in Example 2. Absorber fluid, volume fraction of
the absorber fluid and build layer thickness and were the same as
in Example 1. After printing one layer of absorbent, printed area
was eliminated by Red Photon Engine LED that has radiant power of
1000 mW, obtained from Teledyne Electronic Technologies. Material
has not sintered and the surface temperature increased only to 45
degree Celsius.
Example 6
[0260] Production of a wafer by chemical sintering.
[0261] The thermoplastic filler was CAPA 6501 polycaprolactone from
Solvay Caprolactones. In weigh boat experiments, the binder was
just deionized water. On a ZPrinter.RTM.310, the binder used was
zb.RTM.58 and the binder volume fraction was about 1.13. Printing
was performed with an acidic binder consisting of 90 wt % zb.RTM.58
and 10 wt % glacial acetic acid. Binder volume ratio used for this
experiment was 0.375 (the zp102/zb.RTM.56 saturation).
[0262] It is to be understood that the foregoing embodiments are
presented by way of example only and that, within the scope of the
appended claims and equivalents thereto, the invention may be
practiced otherwise than as specifically described. For example,
many various processes for depositing absorbers over a powdered
substrate are possible, including, but not limited to, extrusion,
electrophotography transfer printing, and spraying through a
stencil.
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